The Better Youtube Firefox extension, is now fully compatible with Firefox 3.Install the better youtube extension and go to any youtube video to see the extension in action.
What Youtube Video Firefox 3 extension does?
Adds a download youtube video link on the youtube page itself.
Replaces default youtube video player with the Flow Player.
Gives a theater effect for the youtube videos by automatically adjusting the video size and takes all the other clutters from the video page.
Download Extension here. (Firefox Only.Requires Firefox restart)
Also checkout these Youtube Hacks