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

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

Index: tools/testing/extensions/firefox/ConsoleCollector/chrome/content/console.js
===================================================================
--- tools/testing/extensions/firefox/ConsoleCollector/chrome/content/console.js	(revision 13269)
+++ tools/testing/extensions/firefox/ConsoleCollector/chrome/content/console.js	(working copy)
@@ -22,12 +22,14 @@
     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;
@@ -72,11 +74,16 @@
     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.
@@ -87,9 +94,6 @@
     }
   };
 
-  // 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);