XUL Add-ons, as browsers with their support, provide to user more freedom in creation and extension of browsers features. XUL has so many features that many addons are impossible to realize in browsers without XUL support.
The entire page will be fading to dark, so you can watch the videos as if you were in the cinema. Works for YouTube™ and Beyond. It works for all known video sites such as YouTube, Vimeo, Dailymotion, Hulu, Metacafe, YouKu, etc.