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 = {};
  
 (function() {
   // An array for collecting the messages.
   var messages = [];
 
   // Add a console message to the collection.
   this.add = function(message) {
     messages.push(message);
   };
 
   // Read the message collection. As a side effect we clear the message list.
   this.read = function(type) {
     var rtn = [];
     for (var i = 0; i < messages.length; i++) {
       var message = messages[i];
-      rtn.push({ 'time' : message.timeStamp,
+      if (message.errorMessage) {
+        rtn.push({ 'time' : message.timeStamp,
                  'source' : message.sourceName,
                  'line': message.lineNumber,
                  'column': message.columnNumber,
                  'category': message.category,
                  'message' : message.errorMessage });
+      }
     }
     messages = [];
     return rtn;
   };
 
   // Clear the message list.
   this.clear = function() {
     messages = [];
   };
 }).apply(ConsoleCollector);
@@ skipping 24 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);
 }());