Worked around bug 924340, updating extension will fail in current Firefox nightlies

lib/contentPolicy.js

 /*
  * This file is part of Adblock Plus <http://adblockplus.org/>,
  * Copyright (C) 2006-2013 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 347 matching lines @@
   classID: Components.ID("cfeaabe6-1dd1-11b2-a0c6-cb5c268894c9"),
   contractID: "@adblockplus.org/abp/policy;1",
   xpcom_categories: ["content-policy", "net-channel-event-sinks"],
 
   /**
    * Registers the content policy on startup.
    */
   init: function()
   {
     let registrar = Components.manager.QueryInterface(Ci.nsIComponentRegistrar);
-    registrar.registerFactory(this.classID, this.classDescription, this.contractID, this);
+    try
+    {
+      registrar.registerFactory(this.classID, this.classDescription, this.contractID, this);
+    }
+    catch (e if e.result == Cr.NS_ERROR_FACTORY_EXISTS)
+    {
+      // See bug 924340 - it might be too early to init now, the old version
+      // we are replacing didn't finish removing itself yet.
+      Utils.runAsync(this.init.bind(this));
+      return;
+    }
 
     let catMan = Utils.categoryManager;
     for each (let category in this.xpcom_categories)
       catMan.addCategoryEntry(category, this.contractID, this.contractID, false, true);
 
     // http-on-opening-request is new in Gecko 18, http-on-modify-request can
     // be used in earlier releases.
     let httpTopic = "http-on-opening-request";
     if (Services.vc.compare(Utils.platformVersion, "18.0") < 0)
       httpTopic = "http-on-modify-request";
@@ skipping 366 matching lines @@
   if (!wnd || wnd.closed)
     return;
 
   if (entry.type == Policy.type.OBJECT)
   {
     node.removeEventListener("mouseover", objectMouseEventHander, true);
     node.removeEventListener("mouseout", objectMouseEventHander, true);
   }
   Policy.processNode(wnd, node, entry.type, Utils.makeURI(entry.location), true);
 }