YouPlayer for Firefox (discovered by Kay) has to be one of best Firefox extensions ever and I am still surprised how I never discovered it until now.
It basically allows you to play videos from your favorite video sites such as YouTube, DailyMotion, MetaCafe, MySpace right from within your Firefox sidebar.
Users can make a playlist of videos and watch them while surfing the web. The reason I love this extension is because it kills the limitation of staying on a browser window to watch a video and allows you to enjoy it virtually anywhere you go.
Once you install this extension (link provided at the end of this post), simply click the red YP button on the bottom right section of Firefox to bring up the YouPlayer sidebar. To start watching videos, all you have to do is drag the video link from your browser to the playlist section and double click it.
And if you still think YouPlayer is not cool enough, let me tell you that YouPlayer also allows you to even drag local files from hard drive into playlist and if you want you can download videos from YouTube by right clicking their link in the playlist and selecting Download.