When applying to windows on install, use front-to-back order (seebug 819561)

lib/windowObserver.js

Index: lib/windowObserver.js
===================================================================
--- a/lib/windowObserver.js
+++ b/lib/windowObserver.js
@@ -30,17 +30,22 @@ exports.WindowObserver = WindowObserver;
  *                          and "end" (default) means to wait until the "load" event.
  * @constructor
  */
 function WindowObserver(listener, when)
 {
   this._listener  = listener;
   this._when = when;
 
-  let e = Services.ww.getWindowEnumerator();
+  let e = Services.wm.getZOrderDOMWindowEnumerator(null, true);
+  if (!e.hasMoreElements())
+  {
+    // On Linux the list returned will be empty, see bug 156333. Fall back to random order.
+    e = Services.wm.getEnumerator(null);
+  }
   while (e.hasMoreElements())
   {
     let window = e.getNext().QueryInterface(Ci.nsIDOMWindow);
     if (when == "start" || window.document.readyState == "complete")
       this._listener.applyToWindow(window);
     else
       this.observe(window, "chrome-document-global-created", null);
   }