diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index 903f04cc79cd..61096e45f502 100644
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -554,14 +554,14 @@ var gPopupBlockerObserver = {
       blockedPopupAllowSite.removeAttribute("disabled");
 
     var foundUsablePopupURI = false;
-    var pageReports = gBrowser.pageReport;
-    if (pageReports) {
-      for (let pageReport of pageReports) {
+    var pageReport = gBrowser.pageReport;
+    if (pageReport) {
+      for (var i = 0; i < pageReport.length; ++i) {
         // popupWindowURI will be null if the file picker popup is blocked.
         // xxxdz this should make the option say "Show file picker" and do it (Bug 590306) 
-        if (!pageReport.popupWindowURI)
+        if (!pageReport[i].popupWindowURI)
           continue;
-        var popupURIspec = pageReport.popupWindowURI.spec;
+        var popupURIspec = pageReport[i].popupWindowURI.spec;
 
         // Sometimes the popup URI that we get back from the pageReport
         // isn't useful (for instance, netscape.com's popup URI ends up
@@ -584,11 +584,11 @@ var gPopupBlockerObserver = {
                                                         [popupURIspec]);
         menuitem.setAttribute("label", label);
         menuitem.setAttribute("popupWindowURI", popupURIspec);
-        menuitem.setAttribute("popupWindowFeatures", pageReport.popupWindowFeatures);
-        menuitem.setAttribute("popupWindowName", pageReport.popupWindowName);
+        menuitem.setAttribute("popupWindowFeatures", pageReport[i].popupWindowFeatures);
+        menuitem.setAttribute("popupWindowName", pageReport[i].popupWindowName);
         menuitem.setAttribute("oncommand", "gPopupBlockerObserver.showBlockedPopup(event);");
-        menuitem.requestingWindow = pageReport.requestingWindow;
-        menuitem.requestingDocument = pageReport.requestingDocument;
+        menuitem.requestingWindow = pageReport[i].requestingWindow;
+        menuitem.requestingDocument = pageReport[i].requestingDocument;
         aEvent.target.appendChild(menuitem);
       }
     }
@@ -692,7 +692,8 @@ const gXPInstallObserver = {
 
   _getBrowser: function (aDocShell)
   {
-    for (let browser of gBrowser.browsers) {
+    for (var i = 0; i < gBrowser.browsers.length; ++i) {
+      var browser = gBrowser.getBrowserAtIndex(i);
       if (this._findChildShell(browser.docShell, aDocShell))
         return browser;
     }
@@ -783,23 +784,23 @@ const gXPInstallObserver = {
       break;
     case "addon-install-failed":
       // TODO This isn't terribly ideal for the multiple failure case
-      for (let install of installInfo.installs) {
-        let host = (installInfo.originatingURI instanceof Ci.nsIStandardURL) &&
+      installInfo.installs.forEach(function(aInstall) {
+        var host = (installInfo.originatingURI instanceof Ci.nsIStandardURL) &&
                    installInfo.originatingURI.host;
         if (!host)
-          host = (install.sourceURI instanceof Ci.nsIStandardURL) &&
-                 install.sourceURI.host;
+          host = (aInstall.sourceURI instanceof Ci.nsIStandardURL) &&
+                 aInstall.sourceURI.host;
 
-        let error = (host || install.error == 0) ? "addonError" : "addonLocalError";
-        if (install.error != 0)
-          error += install.error;
-        else if (install.addon.blocklistState == Ci.nsIBlocklistService.STATE_BLOCKED)
+        var error = (host || aInstall.error == 0) ? "addonError" : "addonLocalError";
+        if (aInstall.error != 0)
+          error += aInstall.error;
+        else if (aInstall.addon.blocklistState == Ci.nsIBlocklistService.STATE_BLOCKED)
           error += "Blocklisted";
         else
           error += "Incompatible";
 
         messageString = gNavigatorBundle.getString(error);
-        messageString = messageString.replace("#1", install.name);
+        messageString = messageString.replace("#1", aInstall.name);
         if (host)
           messageString = messageString.replace("#2", host);
         messageString = messageString.replace("#3", brandShortName);
@@ -807,7 +808,7 @@ const gXPInstallObserver = {
 
         PopupNotifications.show(browser, notificationID, messageString, anchorID,
                                 action, null, options);
-      }
+      });
       break;
     case "addon-install-complete":
       var needsRestart = installInfo.installs.some(function(i) {
@@ -834,14 +835,14 @@ const gXPInstallObserver = {
             // installs
             var types = {};
             var bestType = null;
-            for (let install of installInfo.installs) {
-              if (install.type in types)
-                types[install.type]++;
+            installInfo.installs.forEach(function(aInstall) {
+              if (aInstall.type in types)
+                types[aInstall.type]++;
               else
-                types[install.type] = 1;
-              if (!bestType || types[install.type] > types[bestType])
-                bestType = install.type;
-            }
+                types[aInstall.type] = 1;
+              if (!bestType || types[aInstall.type] > types[bestType])
+                bestType = aInstall.type;
+            });
 
             BrowserOpenAddonsMgr("addons://list/" + bestType);
           }
@@ -1137,7 +1138,7 @@ let gGestureSupport = {
     });
 
     // Try each combination of key presses in decreasing order for commands
-    for (let subCombo of this._power(keyCombos)) {
+    for each (let subCombo in this._power(keyCombos)) {
       // Convert a gesture and pressed keys into the corresponding command
       // action where the preference has the gesture before "shift" before
       // "alt" before "ctrl" before "meta" all separated by periods
@@ -1888,8 +1889,8 @@ function nonBrowserWindowStartup() {
                        'View:PageInfo', 'Tasks:InspectPage', 'Browser:ToggleTabView', 'Browser:ToggleAddonBar'];
   var element;
 
-  for (let disabledItem of disabledItems) {
-    element = document.getElementById(disabledItem);
+  for (var id in disabledItems) {
+    element = document.getElementById(disabledItems[id]);
     if (element)
       element.setAttribute("disabled", "true");
   }
@@ -1898,8 +1899,8 @@ function nonBrowserWindowStartup() {
   // and zoom menu commands as well
   if (window.location.href == "chrome://browser/content/hiddenWindow.xul") {
     var hiddenWindowDisabledItems = ['cmd_close', 'minimizeWindow', 'zoomWindow'];
-    for (let hiddenWindowDisabledItem of hiddenWindowDisabledItems) {
-      element = document.getElementById(hiddenWindowDisabledItem);
+    for (var id in hiddenWindowDisabledItems) {
+      element = document.getElementById(hiddenWindowDisabledItems[id]);
       if (element)
         element.setAttribute("disabled", "true");
     }
@@ -1992,7 +1993,7 @@ function initializeSanitizer()
     if (doMigrate) {
       let cpdBranch = gPrefService.getBranch("privacy.cpd.");
       let clearOnShutdownBranch = gPrefService.getBranch("privacy.clearOnShutdown.");
-      for (let name of itemArray) {
+      itemArray.forEach(function (name) {
         try {
           // don't migrate password or offlineApps clearing in the CRH dialog since
           // there's no UI for those anymore. They default to false. bug 497656
@@ -2003,7 +2004,7 @@ function initializeSanitizer()
         catch(e) {
           Cu.reportError("Exception thrown during privacy pref migration: " + e);
         }
-      }
+      });
     }
 
     gPrefService.setBoolPref("privacy.sanitize.migrateFx3Prefs", true);
@@ -3096,7 +3097,9 @@ function FillInHTMLTooltip(tipElement)
         lookingForSVGTitle = false;
       }
       if (lookingForSVGTitle) {
-        for (let childNode of tipElement.childNodes) {
+        let length = tipElement.childNodes.length;
+        for (let i = 0; i < length; i++) {
+          let childNode = tipElement.childNodes[i];
           if (childNode instanceof SVGTitleElement) {
             SVGTitleText = childNode.textContent;
             break;
@@ -3294,9 +3297,10 @@ const DOMLinkHandler = {
     var feedAdded = false;
     var iconAdded = false;
     var searchAdded = false;
+    var relStrings = rel.split(/\s+/);
     var rels = {};
-    for (let relString of rel.split(/\s+/))
-      rels[relString] = true;
+    for (let i = 0; i < relStrings.length; i++)
+      rels[relStrings[i]] = true;
 
     for (let relVal in rels) {
       switch (relVal) {
@@ -3689,8 +3693,8 @@ function BrowserCustomizeToolbar()
 {
   // Disable the toolbar context menu items
   var menubar = document.getElementById("main-menubar");
-  for (let childNode of menubar.childNodes)
-    childNode.setAttribute("disabled", true);
+  for (var i = 0; i < menubar.childNodes.length; ++i)
+    menubar.childNodes[i].setAttribute("disabled", true);
 
   var cmd = document.getElementById("cmd_CustomizeToolbars");
   cmd.setAttribute("disabled", "true");
@@ -3787,8 +3791,8 @@ function BrowserToolboxCustomizeDone(aToolboxChanged) {
 
   // Re-enable parts of the UI we disabled during the dialog
   var menubar = document.getElementById("main-menubar");
-  for (let childNode of menubar.childNodes)
-    childNode.setAttribute("disabled", false);
+  for (var i = 0; i < menubar.childNodes.length; ++i)
+    menubar.childNodes[i].setAttribute("disabled", false);
   var cmd = document.getElementById("cmd_CustomizeToolbars");
   cmd.removeAttribute("disabled");
 
@@ -4337,37 +4341,39 @@ var FullScreen = {
   {
     var els = document.getElementsByTagNameNS(this._XULNS, aTag);
 
-    for (let el of els) {
+    for (var i = 0; i < els.length; ++i) {
       // XXX don't interfere with previously collapsed toolbars
-      if (el.getAttribute("fullscreentoolbar") == "true") {
+      if (els[i].getAttribute("fullscreentoolbar") == "true") {
         if (!aShow) {
 
-          var toolbarMode = el.getAttribute("mode");
+          var toolbarMode = els[i].getAttribute("mode");
           if (toolbarMode != "text") {
-            el.setAttribute("saved-mode", toolbarMode);
-            el.setAttribute("saved-iconsize", el.getAttribute("iconsize"));
-            el.setAttribute("mode", "icons");
-            el.setAttribute("iconsize", "small");
+            els[i].setAttribute("saved-mode", toolbarMode);
+            els[i].setAttribute("saved-iconsize",
+                                els[i].getAttribute("iconsize"));
+            els[i].setAttribute("mode", "icons");
+            els[i].setAttribute("iconsize", "small");
           }
 
           // Give the main nav bar and the tab bar the fullscreen context menu,
           // otherwise remove context menu to prevent breakage
-          el.setAttribute("saved-context", el.getAttribute("context"));
-          if (el.id == "nav-bar" || el.id == "TabsToolbar")
-            el.setAttribute("context", "autohide-context");
+          els[i].setAttribute("saved-context",
+                              els[i].getAttribute("context"));
+          if (els[i].id == "nav-bar" || els[i].id == "TabsToolbar")
+            els[i].setAttribute("context", "autohide-context");
           else
-            el.removeAttribute("context");
+            els[i].removeAttribute("context");
 
           // Set the inFullscreen attribute to allow specific styling
           // in fullscreen mode
-          el.setAttribute("inFullscreen", true);
+          els[i].setAttribute("inFullscreen", true);
         }
         else {
           function restoreAttr(attrName) {
             var savedAttr = "saved-" + attrName;
-            if (el.hasAttribute(savedAttr)) {
-              el.setAttribute(attrName, el.getAttribute(savedAttr));
-              el.removeAttribute(savedAttr);
+            if (els[i].hasAttribute(savedAttr)) {
+              els[i].setAttribute(attrName, els[i].getAttribute(savedAttr));
+              els[i].removeAttribute(savedAttr);
             }
           }
 
@@ -4375,15 +4381,15 @@ var FullScreen = {
           restoreAttr("iconsize");
           restoreAttr("context");
 
-          el.removeAttribute("inFullscreen");
+          els[i].removeAttribute("inFullscreen");
         }
       } else {
         // use moz-collapsed so it doesn't persist hidden/collapsed,
         // so that new windows don't have missing toolbars
         if (aShow)
-          el.removeAttribute("moz-collapsed");
+          els[i].removeAttribute("moz-collapsed");
         else
-          el.setAttribute("moz-collapsed", "true");
+          els[i].setAttribute("moz-collapsed", "true");
       }
     }
 
@@ -4418,8 +4424,8 @@ var FullScreen = {
     }
 
     var controls = document.getElementsByAttribute("fullscreencontrol", "true");
-    for (let control of controls)
-      control.hidden = aShow;
+    for (var i = 0; i < controls.length; ++i)
+      controls[i].hidden = aShow;
   }
 };
 XPCOMUtils.defineLazyGetter(FullScreen, "useLionFullScreen", function() {
@@ -4553,10 +4559,9 @@ var XULBrowserWindow = {
     if (this._busyUI)
       types.push("status");
     types.push("jsStatus", "jsDefaultStatus", "defaultStatus");
-    for (type of types) {
+    for (let i = 0; !text && i < types.length; i++) {
+      type = types[i];
       text = this[type];
-      if (text)
-        break;
     }
 
     // check the current value so we don't trigger an attribute change
@@ -4819,12 +4824,12 @@ var XULBrowserWindow = {
       let findCommands = [document.getElementById("cmd_find"),
                           document.getElementById("cmd_findAgain"),
                           document.getElementById("cmd_findPrevious")];
-      for (let elt of findCommands) {
+      findCommands.forEach(function (elt) {
         if (disableFind)
           elt.setAttribute("disabled", "true");
         else
           elt.removeAttribute("disabled");
-      }
+      });
 
       if (gFindBarInitialized) {
         if (gFindBar.findMode != gFindBar.FIND_NORMAL) {
@@ -5336,8 +5341,8 @@ function onViewToolbarsPopupShowing(aEvent, aInsertPoint) {
   let toolbarNodes = Array.slice(gNavToolbox.childNodes);
   toolbarNodes.push(document.getElementById("addon-bar"));
 
-  for (let toolbar of toolbarNodes) {
-    let toolbarName = toolbar.getAttribute("toolbarname");
+  toolbarNodes.forEach(function(toolbar) {
+    var toolbarName = toolbar.getAttribute("toolbarname");
     if (toolbarName) {
       let menuItem = document.createElement("menuitem");
       let hidingAttribute = toolbar.getAttribute("type") == "menubar" ?
@@ -5356,7 +5361,7 @@ function onViewToolbarsPopupShowing(aEvent, aInsertPoint) {
 
       menuItem.addEventListener("command", onViewToolbarCommand, false);
     }
-  }
+  }, this);
 }
 
 function onViewToolbarCommand(aEvent) {
@@ -5630,14 +5635,14 @@ function toggleSidebar(commandID, forceOpen) {
 
   // ..but first update the 'checked' state of all sidebar broadcasters
   var broadcasters = document.getElementsByAttribute("group", "sidebar");
-  for (let broadcaster of broadcasters) {
+  for (var i = 0; i < broadcasters.length; ++i) {
     // skip elements that observe sidebar broadcasters and random
     // other elements
-    if (broadcaster.localName != "broadcaster")
+    if (broadcasters[i].localName != "broadcaster")
       continue;
 
-    if (broadcaster != sidebarBroadcaster)
-      broadcaster.removeAttribute("checked");
+    if (broadcasters[i] != sidebarBroadcaster)
+      broadcasters[i].removeAttribute("checked");
     else
       sidebarBroadcaster.setAttribute("checked", "true");
   }
@@ -6167,7 +6172,9 @@ var gPageStyleMenu = {
     var haveAltSheets = false;
     var altStyleSelected = false;
 
-    for (let currentStyleSheet of styleSheets) {
+    for (let i = 0; i < styleSheets.length; ++i) {
+      let currentStyleSheet = styleSheets[i];
+
       if (!currentStyleSheet.title)
         continue;
 
@@ -6376,9 +6383,9 @@ var OfflineApps = {
     // This depends on pseudo APIs of browser.js and tabbrowser.xml
     aContentWindow = aContentWindow.top;
     var browsers = aBrowserWindow.gBrowser.browsers;
-    for (let browser of browsers) {
-      if (browser.contentWindow == aContentWindow)
-        return browser;
+    for (var i = 0; i < browsers.length; ++i) {
+      if (browsers[i].contentWindow == aContentWindow)
+        return browsers[i];
     }
     return null;
   },
@@ -6409,10 +6416,10 @@ var OfflineApps = {
     }
 
     var browsers = gBrowser.browsers;
-    for (let browser of browsers) {
-      uri = this._getManifestURI(browser.contentWindow);
+    for (var i = 0; i < browsers.length; ++i) {
+      uri = this._getManifestURI(browsers[i].contentWindow);
       if (uri && uri.equals(aCacheUpdate.manifestURI)) {
-        return browser;
+        return browsers[i];
       }
     }
 
@@ -6458,10 +6465,10 @@ var OfflineApps = {
       groups = cacheService.getGroups();
 
     var usage = 0;
-    for (let group of groups) {
-      var uri = Services.io.newURI(group, null, null);
+    for (var i = 0; i < groups.length; i++) {
+      var uri = Services.io.newURI(groups[i], null, null);
       if (uri.asciiHost == host) {
-        var cache = cacheService.getActiveCache(group);
+        var cache = cacheService.getActiveCache(groups[i]);
         usage += cache.usage;
       }
     }
@@ -6523,16 +6530,16 @@ var OfflineApps = {
         label: gNavigatorBundle.getString("offlineApps.allow"),
         accessKey: gNavigatorBundle.getString("offlineApps.allowAccessKey"),
         callback: function() {
-          for (let document of notification.documents) {
-            OfflineApps.allowSite(document);
+          for (var i = 0; i < notification.documents.length; i++) {
+            OfflineApps.allowSite(notification.documents[i]);
           }
         }
       },{
         label: gNavigatorBundle.getString("offlineApps.never"),
         accessKey: gNavigatorBundle.getString("offlineApps.neverAccessKey"),
         callback: function() {
-          for (let document of notification.documents) {
-            OfflineApps.disallowSite(document);
+          for (var i = 0; i < notification.documents.length; i++) {
+            OfflineApps.disallowSite(notification.documents[i]);
           }
         }
       },{
@@ -6776,16 +6783,19 @@ function WindowIsClosing()
     return false;
   }
 
-  if (!closeWindow(false, warnAboutClosingWindow))
+  var reallyClose = closeWindow(false, warnAboutClosingWindow);
+  if (!reallyClose)
     return false;
 
-  for (let browser of gBrowser.browsers) {
-    let ds = browser.docShell;
+  var numBrowsers = gBrowser.browsers.length;
+  for (let i = 0; reallyClose && i < numBrowsers; ++i) {
+    let ds = gBrowser.browsers[i].docShell;
+
     if (ds.contentViewer && !ds.contentViewer.permitUnload())
-      return false;
+      reallyClose = false;
   }
 
-  return true;
+  return reallyClose;
 }
 
 /**
@@ -6924,6 +6934,7 @@ function AddKeywordForSearchField() {
                                                   [node.ownerDocument.title]);
   var description = PlacesUIUtils.getDescriptionFromDocument(node.ownerDocument);
 
+  var el, type;
   var formData = [];
 
   function escapeNameValuePair(aName, aValue, aIsFormUrlEncoded) {
@@ -6933,7 +6944,9 @@ function AddKeywordForSearchField() {
       return escape(aName) + "=" + escape(aValue);
   }
 
-  for (let el of node.form.elements) {
+  for (var i=0; i < node.form.elements.length; i++) {
+    el = node.form.elements[i];
+
     if (!el.type) // happens with fieldsets
       continue;
 
@@ -6944,7 +6957,7 @@ function AddKeywordForSearchField() {
       continue;
     }
 
-    let type = el.type.toLowerCase();
+    type = el.type.toLowerCase();
 
     if (((el instanceof HTMLInputElement && el.mozIsTextField(true)) ||
         type == "hidden" || type == "textarea") ||
@@ -7633,7 +7646,8 @@ var FeedHandler = {
       return false;
 
     // Build the menu showing the available feed choices for viewing.
-    for (let feedInfo of feeds) {
+    for (var i = 0; i < feeds.length; ++i) {
+      var feedInfo = feeds[i];
       var menuItem = document.createElement("menuitem");
       var baseTitle = feedInfo.title || feedInfo.href;
       var labelStr = gNavigatorBundle.getFormattedString("feedShowFeedNew", [baseTitle]);
@@ -8856,7 +8870,9 @@ var LightWeightThemeWebInstaller = {
 function switchToTabHavingURI(aURI, aOpenNew) {
   // This will switch to the tab in aWindow having aURI, if present.
   function switchIfURIInWindow(aWindow) {
-    for (let browser of aWindow.gBrowser.browsers) {
+    let browsers = aWindow.gBrowser.browsers;
+    for (let i = 0; i < browsers.length; i++) {
+      let browser = browsers[i];
       if (browser.currentURI.equals(aURI)) {
         // Focus the matching window & tab
         aWindow.focus();
@@ -8917,13 +8933,13 @@ var TabContextMenu = {
       disabled && gBrowser.tabContainer._closeWindowWithLastTab;
 
     var menuItems = aPopupMenu.getElementsByAttribute("tbattr", "tabbrowser-multiple");
-    for (let menuItem of menuItems)
-      menuItem.disabled = disabled;
+    for (var i = 0; i < menuItems.length; i++)
+      menuItems[i].disabled = disabled;
 
     disabled = gBrowser.visibleTabs.length == 1;
     menuItems = aPopupMenu.getElementsByAttribute("tbattr", "tabbrowser-multiple-visible");
-    for (let menuItem of menuItems)
-      menuItem.disabled = disabled;
+    for (var i = 0; i < menuItems.length; i++)
+      menuItems[i].disabled = disabled;
 
     // Session store
     document.getElementById("context_undoCloseTab").disabled =
@@ -9033,10 +9049,10 @@ let AddonsMgrListener = {
     var defaultOrNoninteractive = this.addonBar.getAttribute("defaultset")
                                       .split(",")
                                       .concat(["separator", "spacer", "spring"]);
-    for (let item of this.addonBar.currentSet.split(",")) {
+    this.addonBar.currentSet.split(",").forEach(function (item) {
       if (defaultOrNoninteractive.indexOf(item) == -1)
         itemCount++;
-    }
+    });
 
     return itemCount;
   },