Today, I struggled loading our flex application. I always got the following error
ReferenceError: Error #1069: Property http://www.adobe.com/2006/flex/mx/internal::topMostIndex not found on _Main_mx_managers_SystemManager and there is no default value.
It take me some time to find the solution: This error is caused by the new deployed version 4.57 of FlashFirebug from November 17, 2012. The newest version 4.58 fix this problem but it is not reviewed and therefore not yet deployed via auto-update. You can get this version from Mozilla and you can install it manually by moving your mouse over the version description and clicking “Add to Firefox” button.