diff --git a/common/content/bookmarks.js b/common/content/bookmarks.js
index 65ec1150..21b4f4cd 100644
--- a/common/content/bookmarks.js
+++ b/common/content/bookmarks.js
@@ -118,7 +118,7 @@ var Bookmarks = Module("bookmarks", {
      * @param {Element} elem A form element for which to add a keyword.
      */
     addSearchKeyword: function addSearchKeyword(elem) {
-        if (elem instanceof HTMLFormElement || elem.form)
+        if (elem instanceof Ci.nsIDOMHTMLFormElement || elem.form)
             var { url, postData, charset } = DOM(elem).formData;
         else
             var [url, postData, charset] = [elem.href || elem.src, null, elem.ownerDocument.characterSet];
diff --git a/common/content/browser.js b/common/content/browser.js
index a1b11109..331b1464 100644
--- a/common/content/browser.js
+++ b/common/content/browser.js
@@ -66,7 +66,7 @@ var Browser = Module("browser", XPCOM(Ci.nsISupportsWeakReference, ModuleBase),
     events: {
         DOMContentLoaded: function onDOMContentLoaded(event) {
             let doc = event.originalTarget;
-            if (doc instanceof HTMLDocument)
+            if (doc instanceof Ci.nsIDOMHTMLDocument)
                 this._triggerLoadAutocmd("DOMLoad", doc);
         },
 
@@ -78,7 +78,7 @@ var Browser = Module("browser", XPCOM(Ci.nsISupportsWeakReference, ModuleBase),
             if (doc instanceof Document)
                 dactyl.initDocument(doc);
 
-            if (doc instanceof HTMLDocument) {
+            if (doc instanceof Ci.nsIDOMHTMLDocument) {
                 if (doc.defaultView.frameElement) {
                     // document is part of a frameset
 
diff --git a/common/content/dactyl.js b/common/content/dactyl.js
index 02ce9bd1..f44efde9 100644
--- a/common/content/dactyl.js
+++ b/common/content/dactyl.js
@@ -600,7 +600,7 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
                     if (win.frameElement)
                         win.frameElement.blur();
                     // Grr.
-                    if (content.document.activeElement instanceof HTMLIFrameElement)
+                    if (content.document.activeElement instanceof Ci.nsIDOMHTMLIFrameElement)
                         content.document.activeElement.blur();
                 });
             }
diff --git a/common/content/events.js b/common/content/events.js
index 98dc0281..253e9676 100644
--- a/common/content/events.js
+++ b/common/content/events.js
@@ -611,7 +611,10 @@ var Events = Module("events", {
             if (!(services.focus.getLastFocusMethod(win) & 0x3000)
                 && events.isContentNode(elem)
                 && !buffer.focusAllowed(elem)
-                && isinstance(elem, [HTMLInputElement, HTMLSelectElement, HTMLTextAreaElement, Window])) {
+                && isinstance(elem, [Ci.nsIDOMHTMLInputElement,
+                                     Ci.nsIDOMHTMLSelectElement,
+                                     Ci.nsIDOMHTMLTextAreaElement,
+                                     Ci.nsIDOMWindow])) {
 
                 if (elem.frameElement)
                     dactyl.focusContent(true);
@@ -673,7 +676,7 @@ var Events = Module("events", {
 
                 // Hack to deal with <BS> and so forth not dispatching input
                 // events
-                if (key && event.originalTarget instanceof HTMLInputElement && !modes.main.passthrough) {
+                if (key && event.originalTarget instanceof Ci.nsIDOMHTMLInputElement && !modes.main.passthrough) {
                     let elem = event.originalTarget;
                     elem.dactylKeyPress = elem.value;
                     util.timeout(function () {
@@ -844,7 +847,7 @@ var Events = Module("events", {
     // access to the real focus target
     // Huh? --djk
     onFocusChange: util.wrapCallback(function onFocusChange(event) {
-        function hasHTMLDocument(win) win && win.document && win.document instanceof HTMLDocument
+        function hasHTMLDocument(win) win && win.document && win.document instanceof Ci.nsIDOMHTMLDocument
         if (dactyl.ignoreFocus)
             return;
 
@@ -861,7 +864,7 @@ var Events = Module("events", {
             if (elem && elem.readOnly)
                 return;
 
-            if (isinstance(elem, [HTMLEmbedElement, HTMLEmbedElement])) {
+            if (isinstance(elem, [Ci.nsIDOMHTMLEmbedElement, Ci.nsIDOMHTMLEmbedElement])) {
                 if (!modes.main.passthrough && modes.main != modes.EMBED)
                     modes.push(modes.EMBED);
                 return;
@@ -965,8 +968,9 @@ var Events = Module("events", {
 
     isInputElement: function isInputElement(elem) {
         return DOM(elem).isEditable ||
-               isinstance(elem, [HTMLEmbedElement, HTMLObjectElement,
-                                 HTMLSelectElement])
+               isinstance(elem, [Ci.nsIDOMHTMLEmbedElement,
+                                 Ci.nsIDOMHTMLObjectElement,
+                                 Ci.nsIDOMHTMLSelectElement])
     },
 
     kill: function kill(event) {
diff --git a/common/content/hints.js b/common/content/hints.js
index a9154e88..abadef48 100644
--- a/common/content/hints.js
+++ b/common/content/hints.js
@@ -337,9 +337,11 @@ var HintSession = Class("HintSession", CommandMode, {
 
                 if (elem.hasAttributeNS(NS, "hint"))
                     [hint.text, hint.showText] = [elem.getAttributeNS(NS, "hint"), true];
-                else if (isinstance(elem, [HTMLInputElement, HTMLSelectElement, HTMLTextAreaElement]))
+                else if (isinstance(elem, [Ci.nsIDOMHTMLInputElement,
+                                           Ci.nsIDOMHTMLSelectElement,
+                                           Ci.nsIDOMHTMLTextAreaElement]))
                     [hint.text, hint.showText] = hints.getInputHint(elem, doc);
-                else if (elem.firstElementChild instanceof HTMLImageElement && /^\s*$/.test(elem.textContent))
+                else if (elem.firstElementChild instanceof Ci.nsIDOMHTMLImageElement && /^\s*$/.test(elem.textContent))
                     [hint.text, hint.showText] = [elem.firstElementChild.alt || elem.firstElementChild.title, true];
                 else
                     hint.text = elem.textContent.toLowerCase();
@@ -349,7 +351,7 @@ var HintSession = Class("HintSession", CommandMode, {
                 let leftPos = Math.max((rect.left + offsetX), offsetX);
                 let topPos  = Math.max((rect.top + offsetY), offsetY);
 
-                if (elem instanceof HTMLAreaElement)
+                if (elem instanceof Ci.nsIDOMHTMLAreaElement)
                     [leftPos, topPos] = this.getAreaOffset(elem, leftPos, topPos);
 
                 hint.span.setAttribute("style", ["display: none; left:", leftPos, "px; top:", topPos, "px"].join(""));
@@ -596,7 +598,7 @@ var HintSession = Class("HintSession", CommandMode, {
                 if (!hint.valid)
                     continue inner;
 
-                if (hint.text == "" && hint.elem.firstChild && hint.elem.firstChild instanceof HTMLImageElement) {
+                if (hint.text == "" && hint.elem.firstChild && hint.elem.firstChild instanceof Ci.nsIDOMHTMLImageElement) {
                     if (!hint.imgSpan) {
                         let rect = hint.elem.firstChild.getBoundingClientRect();
                         if (!rect)
@@ -614,7 +616,7 @@ var HintSession = Class("HintSession", CommandMode, {
 
                 let str = this.getHintString(hintnum);
                 let text = [];
-                if (hint.elem instanceof HTMLInputElement)
+                if (hint.elem instanceof Ci.nsIDOMHTMLInputElement)
                     if (hint.elem.type === "radio")
                         text.push(UTF8(hint.elem.checked ? "⊙" : "○"));
                     else if (hint.elem.type === "checkbox")
@@ -781,7 +783,8 @@ var Hints = Module("hints", {
         this.addMode("i", "Show image",                           function (elem) dactyl.open(elem.src));
         this.addMode("I", "Show image in a new tab",              function (elem) dactyl.open(elem.src, dactyl.NEW_TAB));
 
-        function isScrollable(elem) isinstance(elem, [HTMLFrameElement, HTMLIFrameElement]) ||
+        function isScrollable(elem) isinstance(elem, [Ci.nsIDOMHTMLFrameElement,
+                                                      Ci.nsIDOMHTMLIFrameElement]) ||
             Buffer.isScrollable(elem, 0, true) || Buffer.isScrollable(elem, 0, false);
     },
 
@@ -850,7 +853,7 @@ var Hints = Module("hints", {
         else {
             for (let [, option] in Iterator(options["hintinputs"])) {
                 if (option == "value") {
-                    if (elem instanceof HTMLSelectElement) {
+                    if (elem instanceof Ci.nsIDOMHTMLSelectElement) {
                         if (elem.selectedIndex >= 0)
                             return [elem.item(elem.selectedIndex).text.toLowerCase(), false];
                     }
diff --git a/common/content/mow.js b/common/content/mow.js
index 689d9a79..b87c7fe9 100644
--- a/common/content/mow.js
+++ b/common/content/mow.js
@@ -157,7 +157,7 @@ var MOW = Module("mow", {
                 dactyl.open(event.target.href, where);
             };
 
-            if (event.target instanceof HTMLAnchorElement)
+            if (event.target instanceof Ci.nsIDOMHTMLAnchorElement)
                 switch (DOM.Event.stringify(event)) {
                 case "<LeftMouse>":
                     openLink(dactyl.CURRENT_TAB);
@@ -192,7 +192,7 @@ var MOW = Module("mow", {
         popupshowing: function onPopupShowing(event) {
             let menu = commandline.widgets.contextMenu;
             let enabled = {
-                link: window.document.popupNode instanceof HTMLAnchorElement,
+                link: window.document.popupNode instanceof Ci.nsIDOMHTMLAnchorElement,
                 path: window.document.popupNode.hasAttribute("path"),
                 selection: !window.document.commandDispatcher.focusedWindow.getSelection().isCollapsed
             };