Improving the YouTube experience

YouTube Cinema is a new Firefox extension that will automatically display any YouTube video with all the other page elements removed. It even goes so far as to let you choose from one of six background colors.
In addition to dimming the lights, the extension also has a few tricks up its sleeve, like a toggle between the 4:3 player and the newer 16:9 version. You can also have it default to the high-quality stream if it's available.
One important thing to note: the extension does not work with content that cannot be embedded elsewhere. Attempting to play these videos will simply give you an error screen. This is because the extension does all its handy work by playing the video as an embed in its own special browser tab. To keep this from being a problem, you can simply turn off the feature that automatically opens up new YouTube videos in cinema mode.
Once installed, YouTube Cinema plays YouTube videos with the rest of the screen blacked out, and gives you options to control things like the player size, video quality, and background coloring.