There are some compatibility issues between the latest Flash player and some Firefox add-ons or settings. Could you see whether anything in this article helps: Flash 11.3 doesn't load video in Firefox.

I uninstalled Realplayer and now everything works fine. Don't really use it and will install when realplayer says they fixed it

i had the same problem could not play you tube. tried lots of things and use the free edition of SuperAnitspyware and now works great.