Does what it says on the tin. So far I have tested it on a Twitter livestream, YouTube, and ESPN+. YouTube and ESPN+ worked flawlessly, but Twitter was a little buggy; activating the plugin caused the video to pause, and the top and bottom of the video get cut off (instead of having black bars on the sides).

Annoyingly, it replaces the website's video player with Firefox's generic player, which is sometimes inferior. However, sites with good video players usually also have good theatre modes, so this add-on generally isn't necessary on those sites anyway.

I am always weary of installing relatively obscure extensions because of security concerns with trusting unknown developers. However, I have reviewed the entire source code of this addon and have found nothing suspicious, so I feel safe in recommending it.