diff --git a/common/content/bookmarks.js b/common/content/bookmarks.js
index 381893f9..842fc894 100644
--- a/common/content/bookmarks.js
+++ b/common/content/bookmarks.js
@@ -257,8 +257,9 @@ var Bookmarks = Module("bookmarks", {
         let engine = hasOwnProperty(this.searchEngines, engineName) && this.searchEngines[engineName];
         if (engine && engine.supportsResponseType(responseType))
             var queryURI = engine.getSubmission(query, responseType).uri.spec;
+
         if (!queryURI)
-            return (callback || util.identity)([]);
+            return promises.fail();
 
         function parse(req) JSON.parse(req.responseText)[1].filter(isString);
         return this.makeSuggestions(queryURI, parse, callback);
@@ -271,25 +272,25 @@ var Bookmarks = Module("bookmarks", {
      * @param {string} url The URL to fetch.
      * @param {function(XMLHttpRequest):[string]} parser The function which
      *      parses the response.
+     * @returns {Promise<Array>}
      */
-    makeSuggestions: function makeSuggestions(url, parser, callback) {
-        function process(req) {
+    makeSuggestions: function makeSuggestions(url, parser) {
+        let deferred = Promise.defer();
+
+        let req = util.fetchUrl(url);
+        req.then(function process(req) {
             let results = [];
             try {
                 results = parser(req);
             }
             catch (e) {
-                util.reportError(e);
+                return deferred.reject(e);
             }
-            if (callback)
-                return callback(results);
-            return results;
-        }
+            deferred.resolve(results);
+        }, Cu.reportError);
 
-        let req = util.httpGet(url, callback && process);
-        if (callback)
-            return req;
-        return process(req);
+        promises.oncancel(deferred, r => promises.cancel(req, reason));
+        return deferred.promise;
     },
 
     suggestionProviders: {},
@@ -536,7 +537,7 @@ var Bookmarks = Module("bookmarks", {
             "Delete a bookmark",
             function (args) {
                 if (args.bang)
-                    commandline.input(_("bookmark.prompt.deleteAll") + " ",
+                    commandline.input(_("bookmark.prompt.deleteAll") + " ").then(
                         function (resp) {
                             if (resp && resp.match(/^y(es)?$/i)) {
                                 bookmarks.remove(Object.keys(bookmarkcache.bookmarks));
@@ -628,7 +629,7 @@ var Bookmarks = Module("bookmarks", {
     },
 
     completion: function initCompletion() {
-        completion.bookmark = function bookmark(context, tags, extra = {}) {
+        completion.bookmark = function bookmark(context, tags, extra={}) {
             context.title = ["Bookmark", "Title"];
             context.format = bookmarks.format;
             iter(extra).forEach(function ([k, v]) {
@@ -721,11 +722,12 @@ var Bookmarks = Module("bookmarks", {
 
                 ctxt.hasItems = ctxt.completions.length;
                 ctxt.incomplete = true;
-                ctxt.cache.request = bookmarks.getSuggestions(name, ctxt.filter, function (compl) {
+                ctxt.cache.request = bookmarks.getSuggestions(name, ctxt.filter);
+                ctxt.cache.request.then(function (compl) {
                     ctxt.incomplete = false;
                     ctxt.completions = array.uniq(ctxt.completions.filter(c => compl.indexOf(c) >= 0)
                                                       .concat(compl), true);
-                });
+                }, Cu.reportError);
             });
         };
 
diff --git a/common/content/commandline.js b/common/content/commandline.js
index 1a9ccb17..584ae3bf 100644
--- a/common/content/commandline.js
+++ b/common/content/commandline.js
@@ -1,6 +1,6 @@
 // Copyright (c) 2006-2008 by Martin Stubenschrott <stubenschrott@vimperator.org>
 // Copyright (c) 2007-2011 by Doug Kearns <dougkearns@gmail.com>
-// Copyright (c) 2008-2013 Kris Maglione <maglione.k@gmail.com>
+// Copyright (c) 2008-2014 Kris Maglione <maglione.k@gmail.com>
 //
 // This work is licensed for reuse under an MIT license. Details are
 // given in the LICENSE.txt file included with this file.
@@ -845,7 +845,6 @@ var CommandLine = Module("commandline", {
      * pop at any time to close the prompt.
      *
      * @param {string} prompt The input prompt to use.
-     * @param {function(string)} callback
      * @param {Object} extra
      * @... {function} onChange - A function to be called with the current
      *     input every time it changes.
@@ -856,15 +855,16 @@ var CommandLine = Module("commandline", {
      * @... {string} default - The initial value that will be returned
      *     if the user presses <CR> straightaway. @default ""
      */
-    input: function _input(prompt, callback, extra = {}) {
-        CommandPromptMode(prompt, update({ onSubmit: callback }, extra)).open();
-    },
+    input: promises.withCallbacks(function _input([callback, reject], prompt, extra={}, thing={}) {
+        if (callable(extra))
+            // Deprecated.
+            [callback, extra] = [extra, thing];
+
+        CommandPromptMode(prompt, update({ onSubmit: callback, onCancel: reject }, extra)).open();
+    }),
 
     readHeredoc: function readHeredoc(end) {
-        let args;
-        commandline.inputMultiline(end, function (res) { args = res; });
-        util.waitFor(() => args !== undefined);
-        return args;
+        return util.waitFor(commandline.inputMultiline(end));
     },
 
     /**
@@ -873,10 +873,10 @@ var CommandLine = Module("commandline", {
      * callback with that string as a parameter.
      *
      * @param {string} end
-     * @param {function(string)} callback
+     * @returns {Promise<string>}
      */
     // FIXME: Buggy, especially when pasting.
-    inputMultiline: function inputMultiline(end, callback) {
+    inputMultiline: promises.withCallbacks(function inputMultiline([callback], end) {
         let cmd = this.command;
         let self = {
             end: "\n" + end + "\n",
@@ -902,7 +902,7 @@ var CommandLine = Module("commandline", {
         this._autosizeMultilineInputWidget();
 
         this.timeout(function () { dactyl.focus(this.widgets.multilineInput); }, 10);
-    },
+    }),
 
     get commandMode() this.commandSession && isinstance(modes.main, modes.COMMAND_LINE),
 
@@ -1384,7 +1384,7 @@ var CommandLine = Module("commandline", {
          *      @default {@link #selected}
          * @returns {object}
          */
-        getItem: function getItem(tuple = this.selected)
+        getItem: function getItem(tuple=this.selected)
             tuple && tuple[0] && tuple[0].items[tuple[1]],
 
         /**
@@ -1499,7 +1499,7 @@ var CommandLine = Module("commandline", {
          *      @default false
          *      @private
          */
-        select: function select(idx, count = 1, fromTab = false) {
+        select: function select(idx, count=1, fromTab=false) {
             switch (idx) {
             case this.UP:
             case this.DOWN:
diff --git a/common/content/dactyl.js b/common/content/dactyl.js
index 93b24e0f..a3443e56 100644
--- a/common/content/dactyl.js
+++ b/common/content/dactyl.js
@@ -538,7 +538,7 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
      * @param {boolean} silent Whether the command should be echoed on the
      *     command line.
      */
-    execute: function execute(str, modifiers = {}, silent = false) {
+    execute: function execute(str, modifiers={}, silent=false) {
         // skip comments and blank lines
         if (/^\s*("|$)/.test(str))
             return;
@@ -890,13 +890,13 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
      *     tabs.
      * @returns {boolean}
      */
-    open: function open(urls, params = {}, force = false) {
+    open: function open(urls, params={}, force=false) {
         if (typeof urls == "string")
             urls = dactyl.parseURLs(urls);
 
         if (urls.length > prefs.get("browser.tabs.maxOpenBeforeWarn", 20) && !force)
-            return commandline.input(_("dactyl.prompt.openMany", urls.length) + " ",
-                function (resp) {
+            return commandline.input(_("dactyl.prompt.openMany", urls.length) + " ")
+                .then(function (resp) {
                     if (resp && resp.match(/^y(es)?$/i))
                         dactyl.open(urls, params, true);
                 });
@@ -1184,7 +1184,7 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
             return [];
         }
     },
-    wrapCallback: function wrapCallback(callback, self = this) {
+    wrapCallback: function wrapCallback(callback, self=this) {
         let save = ["forceOpen"];
         let saved = save.map(p => dactyl[p]);
         return function wrappedCallback() {
diff --git a/common/content/editor.js b/common/content/editor.js
index 39e9a857..c7f5d59a 100644
--- a/common/content/editor.js
+++ b/common/content/editor.js
@@ -386,8 +386,8 @@ var Editor = Module("editor", XPCOM(Ci.nsIEditActionListener, ModuleBase), {
         let keepFocus = modes.stack.some(m => isinstance(m.main, modes.COMMAND_LINE));
 
         if (!forceEditing && textBox && textBox.type == "password") {
-            commandline.input(_("editor.prompt.editPassword") + " ",
-                function (resp) {
+            commandline.input(_("editor.prompt.editPassword") + " ")
+                .then(function (resp) {
                     if (resp && resp.match(/^y(es)?$/i))
                         editor.editFieldExternally(true);
                 });
@@ -424,7 +424,7 @@ var Editor = Module("editor", XPCOM(Ci.nsIEditActionListener, ModuleBase), {
         column = 1 + pre.replace(/[^]*\n/, "").length;
 
         let origGroup = DOM(textBox).highlight.toString();
-        let cleanup = util.yieldable(function cleanup(error) {
+        let cleanup = promises.task(function cleanup(error) {
             if (timer)
                 timer.cancel();
 
@@ -443,9 +443,11 @@ var Editor = Module("editor", XPCOM(Ci.nsIEditActionListener, ModuleBase), {
                 DOM(textBox).highlight.remove("EditorEditing");
                 if (!keepFocus)
                     dactyl.focus(textBox);
+
                 for (let group in values(blink.concat(blink, ""))) {
                     highlight.highlightNode(textBox, origGroup + " " + group);
-                    yield 100;
+
+                    yield promises.sleep(100);
                 }
             }
         });
diff --git a/common/content/events.js b/common/content/events.js
index 2460e6ed..be6220b1 100644
--- a/common/content/events.js
+++ b/common/content/events.js
@@ -212,7 +212,7 @@ var Events = Module("events", {
     /**
      * Wraps an event listener to ensure that errors are reported.
      */
-    wrapListener: function wrapListener(method, self = this) {
+    wrapListener: function wrapListener(method, self=this) {
         method.wrapper = wrappedListener;
         wrappedListener.wrapped = method;
         function wrappedListener(event) {
diff --git a/common/content/hints.js b/common/content/hints.js
index 019f753e..79a6fd62 100644
--- a/common/content/hints.js
+++ b/common/content/hints.js
@@ -1,6 +1,6 @@
 // Copyright (c) 2006-2008 by Martin Stubenschrott <stubenschrott@vimperator.org>
 // Copyright (c) 2007-2011 by Doug Kearns <dougkearns@gmail.com>
-// Copyright (c) 2008-2013 Kris Maglione <maglione.k@gmail.com>
+// Copyright (c) 2008-2014 Kris Maglione <maglione.k@gmail.com>
 //
 // This work is licensed for reuse under an MIT license. Details are
 // given in the LICENSE.txt file included with this file.
@@ -12,7 +12,7 @@
 var HintSession = Class("HintSession", CommandMode, {
     get extendedMode() modes.HINTS,
 
-    init: function init(mode, opts = {}) {
+    init: function init(mode, opts={}) {
         init.supercall(this);
 
         if (!opts.window)
@@ -1054,11 +1054,11 @@ var Hints = Module("hints", {
         return null;
     }, //}}}
 
-    open: function open(mode, opts = {}) {
+    open: function open(mode, opts={}) {
         this._extendedhintCount = opts.count;
 
         mappings.pushCommand();
-        commandline.input(["Normal", mode], null, {
+        commandline.input(["Normal", mode], {
             autocomplete: false,
             completer: function (context) {
                 context.compare = () => 0;
diff --git a/common/content/history.js b/common/content/history.js
index 62aacf1b..5b2c6d3f 100644
--- a/common/content/history.js
+++ b/common/content/history.js
@@ -1,6 +1,6 @@
 // Copyright (c) 2006-2008 by Martin Stubenschrott <stubenschrott@vimperator.org>
 // Copyright (c) 2007-2011 by Doug Kearns <dougkearns@gmail.com>
-// Copyright (c) 2008-2013 Kris Maglione <maglione.k@gmail.com>
+// Copyright (c) 2008-2014 Kris Maglione <maglione.k@gmail.com>
 //
 // This work is licensed for reuse under an MIT license. Details are
 // given in the LICENSE.txt file included with this file.
@@ -13,7 +13,7 @@ var History = Module("history", {
 
     get service() services.history,
 
-    get: function get(filter, maxItems, sort = this.SORT_DEFAULT) {
+    get: function get(filter, maxItems, sort=this.SORT_DEFAULT) {
         if (isString(filter))
             filter = { searchTerms: filter };
 
diff --git a/common/content/mappings.js b/common/content/mappings.js
index e6e47681..8109a579 100644
--- a/common/content/mappings.js
+++ b/common/content/mappings.js
@@ -187,7 +187,7 @@ var MapHive = Class("MapHive", Contexts.Hive, {
      * @param {Object} extra An optional extra configuration hash.
      * @optional
      */
-    add: function (modes, keys, description, action, extra = {}) {
+    add: function (modes, keys, description, action, extra={}) {
         modes = Array.concat(modes);
         if (!modes.every(util.identity))
             throw TypeError(/*L*/"Invalid modes: " + modes);
@@ -817,7 +817,7 @@ var Mappings = Module("mappings", {
         });
     },
     completion: function initCompletion(dactyl, modules, window) {
-        completion.userMapping = function userMapping(context, modes_ = [modes.NORMAL], hive = mappings.user) {
+        completion.userMapping = function userMapping(context, modes_=[modes.NORMAL], hive=mappings.user) {
             context.keys = { text: function (m) m.names[0],
                              description: function (m) m.description + ": " + m.action };
             context.completions = hive.iterate(modes_);
diff --git a/common/content/marks.js b/common/content/marks.js
index cfa27bf3..8bb37916 100644
--- a/common/content/marks.js
+++ b/common/content/marks.js
@@ -1,6 +1,6 @@
 // Copyright (c) 2006-2008 by Martin Stubenschrott <stubenschrott@vimperator.org>
 // Copyright (c) 2007-2011 by Doug Kearns <dougkearns@gmail.com>
-// Copyright (c) 2008-2013 Kris Maglione <maglione.k@gmail.com>
+// Copyright (c) 2008-2014 Kris Maglione <maglione.k@gmail.com>
 //
 // This work is licensed for reuse under an MIT license. Details are
 // given in the LICENSE.txt file included with this file.
@@ -34,7 +34,7 @@ var Marks = Module("marks", {
 
     get localURI() buffer.focusedFrame.document.documentURI.replace(/#.*/, ""),
 
-    Mark: function Mark(params = {}) {
+    Mark: function Mark(params={}) {
         let win = buffer.focusedFrame;
         let doc = win.document;
 
diff --git a/common/content/modes.js b/common/content/modes.js
index cab9600e..897eacf7 100644
--- a/common/content/modes.js
+++ b/common/content/modes.js
@@ -505,7 +505,7 @@ var Modes = Module("modes", {
         return StackElement;
     })(),
     cacheId: 0,
-    boundProperty: function BoundProperty(desc = {}) {
+    boundProperty: function BoundProperty(desc={}) {
         let id = this.cacheId++;
         let value;
 
diff --git a/common/content/tabs.js b/common/content/tabs.js
index b9674ca9..500d0eea 100644
--- a/common/content/tabs.js
+++ b/common/content/tabs.js
@@ -406,7 +406,7 @@ var Tabs = Module("tabs", {
      * @param {number} count How many tabs to remove.
      * @param {boolean} focusLeftTab Focus the tab to the left of the removed tab.
      */
-    remove: function remove(tab, count = 1, focusLeftTab = false) {
+    remove: function remove(tab, count=1, focusLeftTab=false) {
         let res = this.count > count;
 
         let tabs = this.visibleTabs;
diff --git a/common/modules/base.jsm b/common/modules/base.jsm
index 8543e5da..76a4f5fc 100644
--- a/common/modules/base.jsm
+++ b/common/modules/base.jsm
@@ -161,6 +161,7 @@ defineModule("base", {
 this.lazyRequire("cache", ["cache"]);
 this.lazyRequire("config", ["config"]);
 this.lazyRequire("messages", ["_", "Messages"]);
+this.lazyRequire("promises", ["Task", "promises"]);
 this.lazyRequire("services", ["services"]);
 this.lazyRequire("storage", ["File"]);
 this.lazyRequire("util", ["FailedAssertion", "util"]);
@@ -915,16 +916,16 @@ Class.Memoize = function Memoize(getter, wait)
                         }
                     });
 
-                    util.yieldable(function () {
+                    Task.spawn(function () {
                         let wait;
                         for (var res in getter.call(obj)) {
                             if (wait !== undefined)
-                                yield wait;
+                                yield promises.sleep(wait);
                             wait = res;
                         }
                         Class.replaceProperty(obj, key, res);
                         done = true;
-                    })();
+                    });
 
                     return this[key];
                 };
@@ -1147,7 +1148,7 @@ let stub = Class.Property({
  */
 var ErrorBase = Class("ErrorBase", Error, {
     level: 2,
-    init: function EB_init(message, level = 0) {
+    init: function EB_init(message, level=0) {
         let error = Error(message);
         update(this, error);
         this.stack = error.stack;
@@ -1321,7 +1322,7 @@ var StructBase = Class("StructBase", Array, {
 });
 
 var Timer = Class("Timer", {
-    init: function init(minInterval, maxInterval, callback, self = this) {
+    init: function init(minInterval, maxInterval, callback, self=this) {
         this._timer = services.Timer();
         this.callback = callback;
         this.self = self;
diff --git a/common/modules/buffer.jsm b/common/modules/buffer.jsm
index 6da0ed2b..e196344b 100644
--- a/common/modules/buffer.jsm
+++ b/common/modules/buffer.jsm
@@ -85,10 +85,10 @@ var Buffer = Module("Buffer", {
              * @param {string} pref The name of the preference to return.
              * @returns {Promise<string>}
              */
-            get: promises.withCallback(function get(callback, pref) {
+            get: promises.withCallbacks(function get([resolve], pref) {
                 services.contentPrefs.getByDomainAndName(
                     self.uri.host, pref, self.loadContext,
-                    callback);
+                    resolve);
             }),
 
             /**
@@ -97,10 +97,10 @@ var Buffer = Module("Buffer", {
              * @param {string} pref The preference to set.
              * @param {string} value The value to store.
              */
-            set: promises.withCallback(function set(callback, pref, value) {
+            set: promises.withCallbacks(function set([resolve], pref, value) {
                 services.contentPrefs.set(
                     self.uri.host, pref, value, self.loadContext,
-                    callback);
+                    resolve);
             }),
 
             /**
@@ -108,9 +108,9 @@ var Buffer = Module("Buffer", {
              *
              * @param {string} pref The preference to clear.
              */
-            clear: promises.withCallback(function clear(callback, pref) {
+            clear: promises.withCallbacks(function clear([resolve], pref) {
                 services.contentPrefs.removeByDomainAndName(
-                    self.uri.domain, pref, self.loadContext, callback);
+                    self.uri.domain, pref, self.loadContext, resolve);
             }),
         })),
 
@@ -834,7 +834,7 @@ var Buffer = Module("Buffer", {
      * @param {number} count The multiple of 'scroll' lines to scroll.
      * @optional
      */
-    scrollByScrollSize: function scrollByScrollSize(direction, count = 1) {
+    scrollByScrollSize: function scrollByScrollSize(direction, count=1) {
         let { options } = this.modules;
 
         direction = direction ? 1 : -1;
diff --git a/common/modules/commands.jsm b/common/modules/commands.jsm
index ade668fd..afb54b7e 100644
--- a/common/modules/commands.jsm
+++ b/common/modules/commands.jsm
@@ -155,7 +155,7 @@ var Command = Class("Command", {
      * @param {Args} args The Args object passed to {@link #action}.
      * @param {Object} modifiers Any modifiers to be passed to {@link #action}.
      */
-    execute: function execute(args, modifiers = {}) {
+    execute: function execute(args, modifiers={}) {
         const { dactyl } = this.modules;
 
         let context = args.context;
@@ -558,7 +558,7 @@ var CommandHive = Class("CommandHive", Contexts.Hive, {
      * @param {boolean} replace Replace an existing command of the same name.
      *     @optional
      */
-    add: function add(specs, description, action, extra = {}, replace = false) {
+    add: function add(specs, description, action, extra={}, replace=false) {
         const { commands, contexts } = this.modules;
 
         if (!extra.definedAt)
@@ -597,7 +597,7 @@ var CommandHive = Class("CommandHive", Contexts.Hive, {
         return name;
     },
 
-    _add: function _add(names, description, action, extra = {}, replace = false) {
+    _add: function _add(names, description, action, extra={}, replace=false) {
         const { contexts } = this.modules;
         extra.definedAt = contexts.getCaller(Components.stack.caller.caller);
         return this.add.apply(this, arguments);
@@ -969,7 +969,7 @@ var Commands = Module("commands", {
      *     Args object.
      * @returns {Args}
      */
-    parseArgs: function parseArgs(str, params = {}) {
+    parseArgs: function parseArgs(str, params={}) {
         const self = this;
 
         function getNextArg(str, _keepQuotes=keepQuotes) {
@@ -1777,7 +1777,7 @@ var Commands = Module("commands", {
     }
 });
 
-let quote = function quote(q, list, map = Commands.quoteMap) {
+let quote = function quote(q, list, map=Commands.quoteMap) {
     let re = RegExp("[" + list + "]", "g");
     function quote(str) (q + String.replace(str, re, $0 => ($0 in map ? map[$0] : ("\\" + $0)))
                            + q);
diff --git a/common/modules/completion.jsm b/common/modules/completion.jsm
index 7d4e04f7..3cfddce4 100644
--- a/common/modules/completion.jsm
+++ b/common/modules/completion.jsm
@@ -33,7 +33,7 @@ lazyRequire("template", ["template"]);
  * @constructor
  */
 var CompletionContext = Class("CompletionContext", {
-    init: function cc_init(editor, name = "", offset = 0) {
+    init: function cc_init(editor, name="", offset=0) {
         let self = this;
         if (editor instanceof this.constructor) {
             let parent = editor;
diff --git a/common/modules/config.jsm b/common/modules/config.jsm
index 44d3d252..c460bcb0 100644
--- a/common/modules/config.jsm
+++ b/common/modules/config.jsm
@@ -132,6 +132,7 @@ var ConfigBase = Class("ConfigBase", {
                  "options",
                  "overlay",
                  "prefs",
+                 ["promises", "Promise", "Task", "promises"],
                  "protocol",
                  "sanitizer",
                  "services",
diff --git a/common/modules/io.jsm b/common/modules/io.jsm
index 5d913c51..04cd0dc3 100644
--- a/common/modules/io.jsm
+++ b/common/modules/io.jsm
@@ -15,6 +15,7 @@ defineModule("io", {
 
 lazyRequire("config", ["config"]);
 lazyRequire("contexts", ["Contexts", "contexts"]);
+lazyRequire("promises", ["Promise"]);
 lazyRequire("storage", ["File", "storage"]);
 lazyRequire("styles", ["styles"]);
 lazyRequire("template", ["template"]);
@@ -318,7 +319,7 @@ var IO = Module("io", {
      *     @default ""
      * @returns {File}
      */
-    createTempFile: function createTempFile(ext = "txt", label = "") {
+    createTempFile: function createTempFile(ext="txt", label="") {
         let file = services.directory.get("TmpD", Ci.nsIFile);
         file.append(config.name + label + "." + ext);
         file.createUnique(Ci.nsIFile.NORMAL_FILE_TYPE, octal(666));
@@ -446,26 +447,29 @@ var IO = Module("io", {
 
         let process = services.Process(file.file);
         process.run(false, args.map(String), args.length);
-        try {
-            if (callable(blocking))
-                var timer = services.Timer(
-                    function () {
-                        if (!process.isRunning) {
-                            timer.cancel();
-                            util.trapErrors(blocking, self, process.exitValue);
-                        }
-                    },
-                    100, services.Timer.TYPE_REPEATING_SLACK);
-            else if (blocking)
-                while (process.isRunning)
-                    util.threadYield(false, true);
-        }
-        catch (e) {
-            process.kill();
-            throw e;
+
+        let deferred = Promise.defer();
+
+        if (callable(blocking))
+            // Deprecated.
+            deferred.promise.then(blocking);
+        else if (blocking) {
+            // Deprecated?
+            while (process.isRunning)
+                util.threadYield(false, true);
+            return process.exitValue;
         }
 
-        return process.exitValue;
+        let timer = services.Timer(
+            function () {
+                if (!process.isRunning) {
+                    timer.cancel();
+                    deferred.resolve(process.exitValue);
+                }
+            },
+            100, services.Timer.TYPE_REPEATING_SLACK);
+
+        return deferred.promise;
     },
 
     // TODO: when https://bugzilla.mozilla.org/show_bug.cgi?id=68702 is
diff --git a/common/modules/messages.jsm b/common/modules/messages.jsm
index c4b0dea8..1b65009d 100644
--- a/common/modules/messages.jsm
+++ b/common/modules/messages.jsm
@@ -11,7 +11,7 @@ defineModule("messages", {
 
 var Messages = Module("messages", {
 
-    init: function init(name = "messages") {
+    init: function init(name="messages") {
         let self = this;
         this.name = name;
 
@@ -107,7 +107,7 @@ var Messages = Module("messages", {
         let { Buffer, commands, hints, io, mappings, modes, options, sanitizer } = overlay.activeModules;
         file = io.File(file);
 
-        function properties(base, iter_, prop = "description") iter(function _properties() {
+        function properties(base, iter_, prop="description") iter(function _properties() {
             function key(...args) [base, obj.identifier || obj.name].concat(args).join(".").replace(/[\\:=]/g, "\\$&");
 
             for (var obj in iter_) {
diff --git a/common/modules/promises.jsm b/common/modules/promises.jsm
index 25aba2f5..a1b832f9 100644
--- a/common/modules/promises.jsm
+++ b/common/modules/promises.jsm
@@ -9,10 +9,86 @@ defineModule("promises", {
     require: []
 });
 
+lazyRequire("services", ["services"]);
+
 lazyRequire("resource://gre/modules/Promise.jsm", ["Promise"]);
 lazyRequire("resource://gre/modules/Task.jsm", ["Task"]);
 
+function withCallbacks(fn) {
+    return function wrapper(...args) {
+        let deferred = Promise.defer();
+        function resolve(arg) { deferred.resolve(arg); }
+        function reject(arg)  { deferred.reject(arg); }
+        fn.apply(this, [[resolve, reject, deferred]].concat(args));
+        return deferred.promise;
+    }
+}
+
 var Promises = Module("Promises", {
+    _cancel: WeakMap(),
+
+    /**
+     * Allows promises to be canceled..
+     *
+     * @param {Promise} promise The promise to cancel.
+     * @param {*} arg Argument to be passed to the cancellation
+     * function.
+     */
+    cancel: function cancel(promise, reason) {
+        let cleanup = this._cancel.get(promise);
+        if (cleanup) {
+            cleanup[0](promise);
+            cleanup[1].reject(reason);
+        }
+        this._cancel.delete(promise);
+    },
+
+    /**
+     * Registers a cleanup function for the given deferred promise.
+     *
+     * @param {Deferred} promise The promise to cancel.
+     * @param {function} fn The cleanup function.
+     */
+    oncancel: function oncancel(deferred, fn) {
+        this._cancel.set(deferred.promise, [fn, deferred]);
+    },
+
+    /**
+     * Returns a promise which resolves after a brief delay.
+     */
+    delay: withCallbacks(function delay([accept]) {
+        let { mainThread } = services.threading;
+        mainThread.dispatch(accept, mainThread.DISPATCH_NORMAL);
+    }),
+
+    /**
+     * Returns a promise which resolves with the given argument.
+     */
+    accept: function fail(arg) {
+        let deferred = Promise.defer();
+        deferred.resolve(arg);
+        return deferred.promise;
+    },
+
+    /**
+     * Returns a promise which fails with the given argument.
+     */
+    fail: function fail(arg) {
+        let deferred = Promise.defer();
+        deferred.reject(arg);
+        return deferred.promise;
+    },
+
+    /**
+     * Returns a promise which resolves after the given number of
+     * milliseconds.
+     *
+     * @param {number} delay The number of milliseconds to wait.
+     */
+    sleep: withCallbacks(function sleep([callback], delay) {
+        this.timeout(callback, delay);
+    }),
+
     /**
      * Wraps the given function so that each call spawns a Task.
      *
@@ -26,21 +102,45 @@ var Promises = Module("Promises", {
     },
 
     /**
-     * Wraps the given function so that its first argument is a
-     * callback which, when called, resolves the returned promise.
+     * Returns a promise which resolves when the function *test* returns
+     * true, or *timeout* milliseconds have expired.
+     *
+     * @param {function} test The predicate on which to wait.
+     * @param {Number} timeout The maximum number of milliseconds to
+     *      wait.
+     *      @optional
+     * @param {number} pollInterval The poll interval, in milliseconds.
+     *      @default 10
+     */
+    waitFor: withCallbacks(function waitFor([accept, reject], test, timeout=null, pollInterval=10) {
+        let end = timeout && Date.now() + timeout, result;
+
+        let timer = services.Timer(
+            () => {
+                try {
+                    var result = test();
+                }
+                catch (e) {
+                    timer.cancel();
+                    reject(e);
+                }
+                if (result) {
+                    timer.cancel();
+                    accept(result);
+                }
+            },
+            pollInterval, services.Timer.TYPE_REPEATING_SLACK);
+    }),
+
+    /**
+     * Wraps the given function so that its first argument is an array
+     * of success and failure callbacks which, when called, resolve the
+     * returned promise.
      *
      * @param {function} fn The function to wrap.
      * @returns {Promise}
      */
-    withCallback: function withCallback(fn) {
-        return function wrapper(...args) {
-            let deferred = Promise.defer();
-            function callback(arg) {
-                deferred.resolve(arg);
-            }
-            return fn.apply(this, [callback].concat(args));
-        }
-    },
+    withCallbacks: withCallbacks,
 });
 
 endModule();
diff --git a/common/modules/styles.jsm b/common/modules/styles.jsm
index 31756070..12cf60fa 100644
--- a/common/modules/styles.jsm
+++ b/common/modules/styles.jsm
@@ -382,7 +382,7 @@ var Styles = Module("Styles", {
         return val;
     },
 
-    completeSite: function (context, content, group = styles.user) {
+    completeSite: function (context, content, group=styles.user) {
         context.anchored = false;
         try {
             context.fork("current", 0, this, function (context) {
diff --git a/common/modules/template.jsm b/common/modules/template.jsm
index 17c04cf3..4aa2c888 100644
--- a/common/modules/template.jsm
+++ b/common/modules/template.jsm
@@ -467,7 +467,7 @@ var Template = Module("Template", {
                         ["td", { style: style[i] || "" }, d])])];
     },
 
-    usage: function usage(iter, format = {}) {
+    usage: function usage(iter, format={}) {
         let desc = format.description || (item => this.linkifyHelp(item.description));
         let help = format.help || (item => item.name);
         let sourceLink = (frame) => {
diff --git a/common/modules/util.jsm b/common/modules/util.jsm
index 23a4088a..b05fc199 100644
--- a/common/modules/util.jsm
+++ b/common/modules/util.jsm
@@ -10,7 +10,7 @@ try {
 
 defineModule("util", {
     exports: ["DOM", "$", "FailedAssertion", "Math", "NS", "Point", "Util", "XBL", "XHTML", "XUL", "util"],
-    require: ["dom", "services"]
+    require: ["dom", "promises", "services"]
 });
 
 lazyRequire("overlay", ["overlay"]);
@@ -750,10 +750,10 @@ var Util = Module("Util", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
      *
      * @returns {XMLHttpRequest}
      */
-    httpGet: function httpGet(url, callback, self) {
-        let params = callback;
-        if (!isObject(params))
-            params = { callback: params && ((...args) => callback.apply(self, args)) };
+    httpGet: function httpGet(url, params, self) {
+        if (callable(params))
+            // Deprecated.
+            params = { callback: params.bind(self) };
 
         try {
             let xmlhttp = services.Xmlhttp();
@@ -761,8 +761,8 @@ var Util = Module("Util", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
 
             let async = params.callback || params.onload || params.onerror;
             if (async) {
-                xmlhttp.addEventListener("load",  function handler(event) { util.trapErrors(params.onload  || params.callback, params, xmlhttp, event); }, false);
-                xmlhttp.addEventListener("error", function handler(event) { util.trapErrors(params.onerror || params.callback, params, xmlhttp, event); }, false);
+                xmlhttp.addEventListener("load",  event => { util.trapErrors(params.onload  || params.callback, params, xmlhttp, event); }, false);
+                xmlhttp.addEventListener("error", event => { util.trapErrors(params.onerror || params.callback, params, xmlhttp, event); }, false);
             }
 
             if (isObject(params.params)) {
@@ -810,6 +810,22 @@ var Util = Module("Util", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
         }
     },
 
+    /**
+     * Like #httpGet, but returns a promise rather than accepting
+     * callbacks.
+     *
+     * @param {string} url The URL to fetch.
+     * @param {object} params Parameter object, as in #httpGet.
+     */
+    fetchUrl: promises.withCallbacks(function fetchUrl([accept, reject, deferred], url, params) {
+        params = update({}, params);
+        params.onload = accept;
+        params.onerror = reject;
+
+        let req = this.httpGet(url, params);
+        promises.oncancel(deferred, req.cancel);
+    }),
+
     /**
      * The identity function.
      *
@@ -1555,7 +1571,7 @@ var Util = Module("Util", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
      * Waits for the function *test* to return true, or *timeout*
      * milliseconds to expire.
      *
-     * @param {function} test The predicate on which to wait.
+     * @param {function|Promise} test The predicate on which to wait.
      * @param {object} self The 'this' object for *test*.
      * @param {Number} timeout The maximum number of milliseconds to
      *      wait.
@@ -1565,6 +1581,15 @@ var Util = Module("Util", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
      *      thrown.
      */
     waitFor: function waitFor(test, self, timeout, interruptable) {
+        if (!callable(test)) {
+            let done = false;
+            var promise = test,
+                retVal;
+            promise.then((arg) => { retVal = arg; done = true; },
+                         (arg) => { retVal = arg; done = true; });
+            test = () => done;
+        }
+
         let end = timeout && Date.now() + timeout, result;
 
         let timer = services.Timer(function () {}, 10, services.Timer.TYPE_REPEATING_SLACK);
@@ -1575,7 +1600,7 @@ var Util = Module("Util", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
         finally {
             timer.cancel();
         }
-        return result;
+        return promise ? retVal: result;
     },
 
     /**
@@ -1595,7 +1620,7 @@ var Util = Module("Util", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
      * @returns {function} A new function which may not execute
      *      synchronously.
      */
-    yieldable: function yieldable(func)
+    yieldable: deprecated("Task.spawn", function yieldable(func)
         function magic() {
             let gen = func.apply(this, arguments);
             (function next() {
@@ -1604,7 +1629,7 @@ var Util = Module("Util", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
                 }
                 catch (e if e instanceof StopIteration) {};
             })();
-        },
+        }),
 
     /**
      * Wraps a callback function such that its errors are not lost. This