Fixed the issue with the ConsoleCollector extension causing FF to hang if run from WebDriver.

tools/testing/extensions/firefox/ConsoleCollector/chrome/content/console.js

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // This Firefox add-on exposes the Javascript console contents to Javascript
 // running in the browser. Once this is installed there will be a new
 // window.ConsoleCollector object with read() and clear() functions.
 
 var ConsoleCollector = {};
  
@@ skipping 54 matching lines @@
     }
   };
   
   // Start collecting console messages.
   function initialize(event) {
     consoleService = Components.classes['@mozilla.org/consoleservice;1']
                    .getService(Components.interfaces.nsIConsoleService);
     if (consoleService) {
       consoleService.registerListener(consoleListener); 
     }                              
+    // Add the handler for hooking in to each page's DOM. This handler
+    // is for each "gBrowser", representing a tab/window.
+    window.getBrowser().addEventListener("load", onPageLoad, true);
   }
 
   // Stop collecting console messages.
   function shutdown(event) {
+    window.getBrowser().removeEventListener("load", onPageLoad);
     consoleService.unregisterListener(consoleListener);   
+    ConsoleCollector.clear();
   }
   
   // Hook the ConsoleCollector into the DOM as window.ConsoleCollector.
   var onPageLoad = function(e) {
     var win = e.originalTarget.defaultView;
     if (win) {
       win.wrappedJSObject.ConsoleCollector = ConsoleCollector;
     }
   };
 
-  // Add the handler for hooking in to each page's DOM. This handler
-  // is for each "gBrowser", representing a tab/window.
-  window.getBrowser().addEventListener("load", onPageLoad, true);
   // Add the handlers to initialize the add-on and shut it down.
   // These handlers are for the application as a whole.
   window.addEventListener('load', initialize, false);
   window.addEventListener('unload', shutdown, false);
 }());