Issue 4139 - Don't save element hiding filters on disk

lib/elemHideStylesheet.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 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 52 matching lines @@
   {
     onUpdate.inProgress = false;
     apply();
   });
 }
 
 function apply()
 {
   unapply();
 
+  if (!Prefs.enabled)
+    return;
+
   try
   {
     Utils.styleService.loadAndRegisterSheet(styleURL,
         Ci.nsIStyleSheetService.USER_SHEET);
     applied = true;
   }
   catch (e)
   {
     Cu.reportError(e);
   }
@@ skipping 12 matching lines @@
     {
       Cu.reportError(e);
     }
     applied = false;
   }
 }
 
 // Send dummy message before initializing, this delay makes sure that the child
 // modules are loaded and our protocol handler registered.
 port.emitWithResponse("ping").then(init);