This might have been reported before (at least, I know I did it myself before the forum-Ausrottung.)

Often when I quit a game (playing via Firefox), I will end up having a black screen in Windows 7. This can be fixed by ctrl+alt+del'ing. Also, it will often kill whatever part of Windows takes care of the themes and will revert to the default Windows theme until Firefox is closed. This, apparently, is a JS bug (but an annoying bug nonetheless).