I recently had a very frustrating problem with Flashplayer. Whenever I tried to watch any kind of Flash video, the video would black out - and any tab I opened would be blacked out, as well. Eventually, the browser would crash completely. Found a problem about it in Mozilla support, but no solution. Long story short, the problem was AdBlock Plus. (It may have gotten corrupted somehow???) When I completely REMOVED it - restarted FireFox - then reinstalled AdBlock Plus, the problem was fixed.