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

lib/contentPolicy.js

Index: lib/contentPolicy.js
===================================================================
--- a/lib/contentPolicy.js
+++ b/lib/contentPolicy.js
@@ -360,17 +360,27 @@ let PolicyImplementation =
   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";