It's no secret YouTube has a "dimmer switch" that works, and works well. It exists for some long-form partner content like episodes of Star Trek, but is not generally available for others. Services like Hulu have popularized the feature as a nice way to get rid of some of the distracting elements off to the side of the video. In YouTube's case, this is advertisements and the often awful user comments below.
To skip having to wait for YouTube to add it, there's YouTube Cinema, 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. Sure you can have YouTube do this automatically by switching it on in your user profile settings, but if you're watching a video while not logged in or if you don't want to sign up for an account, this is a quick and easy shortcut.
One important thing to note is that this 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.