Try clearing the Cache in Firefox.

Tools-> Clear Recent History -> [v] Cache

or

Tools->Options->Advanced->Network -> Offline storage -> [Clear Now]

Youtube may have made some changes recently. Clear the Youtube cookie and refresh page if above does not do it.

well i tried all the above still didnt work you know what it was? the flash player the newest one dont work go aversion before it works fine so adbode wants to sort there shit flash player out ok thanks for trying to help ididnt think it was firefox from the start i had a ickling it was the flash player.

It may also be a issue with Youtube as people with other browsers are getting the same. http://www.google.com/support/forum/p/youtube/thread?hl=en&tid=3cda4dac122b8f13

Another tip in case what I posted above does not work for a video is to make the youtube url as https:// instead of http://