Invisible Youtube loading bar for Firefox 5.0.1 (9/14, it fixed itself?)

Update plugins (specially flash) -> http://www.mozilla.com/en-US/plugincheck

Then try again. If that doesn't work, restart.

If that doesn't work, wait for the pros.

edit: You could also try disabling the addons, see if any of them is screwing stuff up. Specially those youtube -vid/music ripping- addons and other stuff related to youtube.

I believe addons carry on to newer versions of FF when updating, so maybe that's why updating FF did not get rid of the problem?
-random guess-