Youplayer is a Firefox add-on that enables you to play Youtube videos on your sidebar. In this way, you don't need to stay on Youtube and you can continue browsing different websites while still enjoying your favorite videos.
Youplayer opens single Youtube pages and plays the videos in a small top right window. It also supports playlist management (in the standard M3U format) and even includes a small search field from which to look for videos on Youtube, although the results open up in a new tab –it would be probably better if they opened in the sidebar itself.
A very convenient way to watch Youtube videos while you keep on browsing with Firefox.