The problem conceivably could be with codecs or video card drivers that are no longer compatible and need updating. A temporary workaround may be to try older versions of Firefox or FlashPlayer. There are possibly other related posts such as this one.

You may also find information on the Adobe forums. Another consideration is FlashPlayer's use of hardware acceleration which you may need to turn on or off for better compatability. That is likely to be discussed on youtube/google's own forums and the Adobe Forums such as this post

Clear the cache and the cookies from sites that cause problems.

• "Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites causing problems: Tools > Options > Privacy > Cookies: "Show Cookies"

You can also try to disable the hardware acceleration in the Flash Player.

See Flash videos won't play full screen

• http://www.youtube.com/swf_test.html (right-click the player: Settings)

Flash "Display settings" window:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help01.html