Videos won't play on Firefox

I have windows vista home edition, 64 bit; I have both IE 9 and Firefox although I mainly use Firefox.  I downloaded the latest flash player (11.9.900.170) to both IE and Firefox; the plug-ins are active.  Flash player plays on IE but not on Firefox; on youtube nothing displays; there is either a white space or it says the "video is no longer available"

I've removed older versions of flash player and reinstalled; that didn't work; I've removed firefox and reinstalled; that didn't work either; I always seem to have a problem with every new flash player update and firefox.

What am I doing wrong?