I am getting a pop-up window on several computers now, when I try to shut down the computer. It says something to the effect "xcom event.exe". This happens infrequently, but two different computers are now doing it after version 2.0 was installed. It only happens if I have used Firefox at least once before shutting down.
I remember reading somewhere that FF's XPCOM interface sometimes caused this problem on systems running McAfee antivirus, but IIRC it happened even before v2 was released.
I suppose the best bet is the standard advice - back up your bookmarks, uninstall it completely and delete your profile folder, then reinstall adding any extensions one at a time...