Index: lib/appSupport.js
===================================================================
--- a/lib/appSupport.js
+++ b/lib/appSupport.js
@@ -876,7 +876,7 @@
     onShutdown.add(function()
     {
       let window = null;
-      for (window in UI.applicationWindows)
+      for (window of UI.applicationWindows)
         break;
 
       if (window && menuItem)
Index: lib/ui.js
===================================================================
--- a/lib/ui.js
+++ b/lib/ui.js
@@ -409,7 +409,7 @@
         this.updateState();
       else if (name == "showinstatusbar")
       {
-        for (let window in this.applicationWindows)
+        for (let window of this.applicationWindows)
           this.updateStatusbarIcon(window);
       }
     }.bind(this));
@@ -1085,7 +1085,7 @@
    */
   updateState: function()
   {
-    for (let window in this.applicationWindows)
+    for (let window of this.applicationWindows)
     {
       this.updateIconState(window, window.document.getElementById("abp-status"));
       this.updateIconState(window, window.document.getElementById("abp-toolbarbutton"));
@@ -1966,7 +1966,7 @@
 
 onShutdown.add(function()
 {
-  for (let window in UI.applicationWindows)
+  for (let window of UI.applicationWindows)
     if (UI.isBottombarOpen(window))
       UI.toggleBottombar(window);
 });