diff --git a/addon-sdk/source/lib/sdk/window/utils.js b/addon-sdk/source/lib/sdk/window/utils.js
index 155d6842b3267d..78de9cb3f05e5a 100644
--- a/addon-sdk/source/lib/sdk/window/utils.js
+++ b/addon-sdk/source/lib/sdk/window/utils.js
@@ -297,6 +297,7 @@ function windows(type, options) {
     let window = winEnum.getNext().QueryInterface(Ci.nsIDOMWindow);
     // Only add non-private windows when pb permission isn't set,
     // unless an option forces the addition of them.
+    // XXXbz should this be checking window.closed?
     if (options.includePrivate || !isWindowPrivate(window)) {
       list.push(window);
     }
diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index e30b1f90e3bdfa..80d5b93b070389 100644
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -2177,6 +2177,9 @@ function BrowserPageInfo(doc, initialTab, imageElement) {
   // Check for windows matching the url
   while (windows.hasMoreElements()) {
     var currentWindow = windows.getNext();
+    if (currentWindow.closed) {
+      continue;
+    }
     if (currentWindow.document.documentElement.getAttribute("relatedUrl") == documentURL) {
       currentWindow.focus();
       currentWindow.resetPageInfo(args);
@@ -6040,7 +6043,7 @@ function warnAboutClosingWindow() {
   let nonPopupPresent = false;
   while (e.hasMoreElements()) {
     let win = e.getNext();
-    if (win != window) {
+    if (!win.closed && win != window) {
       if (isPBWindow && PrivateBrowsingUtils.isWindowPrivate(win))
         otherPBWindowExists = true;
       if (win.toolbar.visible)
diff --git a/browser/base/content/utilityOverlay.js b/browser/base/content/utilityOverlay.js
index 0aecd6dd4242e3..ffc116e9013a2c 100644
--- a/browser/base/content/utilityOverlay.js
+++ b/browser/base/content/utilityOverlay.js
@@ -454,6 +454,9 @@ function openAboutDialog() {
   while (enumerator.hasMoreElements()) {
     // Only open one about window (Bug 599573)
     let win = enumerator.getNext();
+    if (win.closed) {
+      continue;
+    }
     win.focus();
     return;
   }
diff --git a/browser/components/nsBrowserGlue.js b/browser/components/nsBrowserGlue.js
index f9a7fe13a505ce..2423b0cfc08319 100644
--- a/browser/components/nsBrowserGlue.js
+++ b/browser/components/nsBrowserGlue.js
@@ -769,6 +769,7 @@ BrowserGlue.prototype = {
     var browserEnum = Services.wm.getEnumerator("navigator:browser");
     let allWindowsPrivate = true;
     while (browserEnum.hasMoreElements()) {
+      // XXXbz should we skip closed windows here?
       windowcount++;
 
       var browser = browserEnum.getNext();
diff --git a/browser/devtools/webconsole/webconsole.js b/browser/devtools/webconsole/webconsole.js
index bc0424520ea588..38b7b5959bfb5b 100644
--- a/browser/devtools/webconsole/webconsole.js
+++ b/browser/devtools/webconsole/webconsole.js
@@ -2608,7 +2608,7 @@ WebConsoleFrame.prototype = {
         while (wins.hasMoreElements()) {
           let win = wins.getNext();
 
-          if (win.Scratchpad.uniqueName === aSourceURL) {
+          if (!win.closed && win.Scratchpad.uniqueName === aSourceURL) {
             win.focus();
             return;
           }
diff --git a/browser/modules/webappsUI.jsm b/browser/modules/webappsUI.jsm
index 3a4c8aea150669..a915733190c901 100644
--- a/browser/modules/webappsUI.jsm
+++ b/browser/modules/webappsUI.jsm
@@ -81,6 +81,9 @@ this.webappsUI = {
     let found = false;
     while (!found && browserEnumerator.hasMoreElements()) {
       let browserWin = browserEnumerator.getNext();
+      if (browserWin.closed) {
+        continue;
+      }
       let tabbrowser = browserWin.gBrowser;
 
       // Check each tab of this browser instance
diff --git a/mobile/android/chrome/content/browser.js b/mobile/android/chrome/content/browser.js
index e673cb1fbbbd3b..0f505e9336c8b2 100644
--- a/mobile/android/chrome/content/browser.js
+++ b/mobile/android/chrome/content/browser.js
@@ -919,7 +919,7 @@ var BrowserApp = {
     let e = Services.wm.getEnumerator("navigator:browser");
     while (e.hasMoreElements() && lastBrowser) {
       let win = e.getNext();
-      if (win != window)
+      if (!win.closed && win != window)
         lastBrowser = false;
     }
 
diff --git a/toolkit/components/help/content/contextHelp.js b/toolkit/components/help/content/contextHelp.js
index cd66615c90e3a9..9c8b6d4a13893a 100644
--- a/toolkit/components/help/content/contextHelp.js
+++ b/toolkit/components/help/content/contextHelp.js
@@ -61,7 +61,7 @@ function locateHelpWindow(contentPack) {
 # pack.
     while (iterator.hasMoreElements()) {
         aWindow = iterator.getNext();
-        if (aWindow.getHelpFileURI() == contentPack) {
+        if (!aWindow.closed && aWindow.getHelpFileURI() == contentPack) {
             topWindow = aWindow;
         }
     }
diff --git a/toolkit/components/social/MozSocialAPI.jsm b/toolkit/components/social/MozSocialAPI.jsm
index 65e7b9b1eb460b..599bb4b38d4f9f 100644
--- a/toolkit/components/social/MozSocialAPI.jsm
+++ b/toolkit/components/social/MozSocialAPI.jsm
@@ -302,7 +302,7 @@ function findChromeWindowForChats(preferredWindow) {
   }
   while (enumerator.hasMoreElements()) {
     let win = enumerator.getNext();
-    if (win && isWindowGoodForChats(win))
+    if (!win.closed && isWindowGoodForChats(win))
       topMost = win;
   }
   return topMost;
diff --git a/toolkit/content/globalOverlay.js b/toolkit/content/globalOverlay.js
index 2665df12e8996f..0c964a2811cf0a 100644
--- a/toolkit/content/globalOverlay.js
+++ b/toolkit/content/globalOverlay.js
@@ -13,6 +13,9 @@ function closeWindow(aClose, aPromptFunction)
   
   while (e.hasMoreElements()) {
     var w = e.getNext();
+    if (w.closed) {
+      continue;
+    }
     if (++windowCount == 2) 
       break;
   }
diff --git a/toolkit/mozapps/extensions/content/extensions.js b/toolkit/mozapps/extensions/content/extensions.js
index 6860e5183d54d5..7161bb009ca0de 100644
--- a/toolkit/mozapps/extensions/content/extensions.js
+++ b/toolkit/mozapps/extensions/content/extensions.js
@@ -927,6 +927,9 @@ var gViewController = {
         var windows = Services.wm.getEnumerator(null);
         while (windows.hasMoreElements()) {
           var win = windows.getNext();
+          if (win.closed) {
+            continue;
+          }
           if (win.document.documentURI == optionsURL) {
             win.focus();
             return;
diff --git a/xpfe/appshell/public/nsIWindowMediator.idl b/xpfe/appshell/public/nsIWindowMediator.idl
index 1114bec9122639..d5c1946b6503d9 100644
--- a/xpfe/appshell/public/nsIWindowMediator.idl
+++ b/xpfe/appshell/public/nsIWindowMediator.idl
@@ -29,7 +29,9 @@ interface nsIWindowMediator: nsISupports
     *                     windows of this type. ("type" is the
     *                     |windowtype| attribute of the XML <window> element.)
     *                     If null, all windows will be enumerated.
-    * @return an enumerator of nsIDOMWindows
+    * @return an enumerator of nsIDOMWindows.  Note that windows close
+    *         asynchronously in many cases, so windows returned from this
+    *         enumerator can have .closed set to true.  Caveat enumerator!
     */
   nsISimpleEnumerator getEnumerator(in wstring aWindowType);