JavaScript YouTube Cinema Mode Button Thanks to luk3yx (for button) and YouTube (For YouTube videos).

This link needs to be added to your bookmarks toolbar.

YouTube Cinema
In most browsers (including Firefox), you can just drag the link to the bookmarks toolbar.
In Firefox, you can also right-click the link, then click "Bookmark This Page". Select 'Bookmarks Toolbar' (or another place), and you are all set.