diff --git a/common/content/dactyl.js b/common/content/dactyl.js
index e4c48d04..2d932031 100644
--- a/common/content/dactyl.js
+++ b/common/content/dactyl.js
@@ -1209,7 +1209,7 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
      * @property {[Window]} Returns an array of all the host application's
      *     open windows.
      */
-    get windows() [win for (win in iter(services.windowMediator.getEnumerator("navigator:browser"))) if (win.dactyl)],
+    get windows() [w for (w of overlay.windows)]
 
 }, {
     toolbarHidden: function hidden(elem) (elem.getAttribute("autohide") || elem.getAttribute("collapsed")) == "true"
@@ -1591,7 +1591,7 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
             function (args) {
                 if (dactyl.has("tabs") && tabs.remove(tabs.getTab(), 1, false))
                     return;
-                else if (dactyl.windows.length > 1)
+                else if (dactyl.windows.size > 1)
                     window.close();
                 else
                     dactyl.quit(false, args.bang);
diff --git a/common/modules/base.jsm b/common/modules/base.jsm
index 39844289..4332683d 100644
--- a/common/modules/base.jsm
+++ b/common/modules/base.jsm
@@ -378,6 +378,9 @@ function values(obj) iter(function values() {
     else if (isinstance(obj, ["Generator", "Iterator", Iter]))
         for (let k in obj)
             yield k;
+    else if (iter.iteratorProp in obj)
+        for (let v of obj)
+            yield v;
     else
         for (var k in obj)
             if (hasOwnProperty(obj, k))
diff --git a/common/modules/main.jsm b/common/modules/main.jsm
index 13fd5aba..1c067f90 100644
--- a/common/modules/main.jsm
+++ b/common/modules/main.jsm
@@ -231,7 +231,7 @@ overlay.overlayWindow(Object.keys(config.overlays),
     },
 
     cleanup: function cleanup(window) {
-        overlay.windows = overlay.windows.filter(w => w != window);
+        overlay.windows.remove(window);
 
         Cu.nukeSandbox(this.jsmodules);
     },
@@ -252,7 +252,7 @@ overlay.overlayWindow(Object.keys(config.overlays),
 
         defineModule.loadLog.push("Loaded in " + (Date.now() - this.startTime) + "ms");
 
-        overlay.windows = array.uniq(overlay.windows.concat(window), true);
+        overlay.windows.add(window);
     },
 
     loadModule: function loadModule(module, prereq, frame) {
diff --git a/common/modules/overlay.jsm b/common/modules/overlay.jsm
index b1f1cab7..ae55eacb 100644
--- a/common/modules/overlay.jsm
+++ b/common/modules/overlay.jsm
@@ -404,19 +404,22 @@ var Overlay = Module("Overlay", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReferen
 
     get activeModules() this.activeWindow && this.activeWindow.dactyl.modules,
 
-    get modules() this.windows.map(w => w.dactyl.modules),
+    get modules() [w.dactyl.modules for (w of this.windows)],
 
     /**
      * The most recently active dactyl window.
      */
-    get activeWindow() this.windows[0],
+    get activeWindow() {
+        let win = this._activeWindow && this._activeWindow.get();
+        return this.windows.has(win) && win;
+    },
 
-    set activeWindow(win) this.windows = [win].concat(this.windows.filter(w => w != win)),
+    set activeWindow(win) this._activeWindow = util.weakReference(win),
 
     /**
      * A list of extant dactyl windows.
      */
-    windows: Class.Memoize(() => [])
+    windows: Class.Memoize(() => RealSet())
 });
 
 endModule();