YouPlayer is a Firefox add-on which adds a lateral bar featuring a video player and a playlist. There, you'll be able to watch those video you usually see. Enjoy the double because now you'll be able to be watching two things at the same time.
It is not annoying while you surf, and you can create playlists, so you'll see what you like the most while you enjoy the magic of internet.
If you are a YouTube fan, YouPlayer is a must have for you and your computer.