No longer works, I am now getting black boxes on top and below of the video player while using theater mode.

To confirm I did reset my PC, as well as restart firefox to confirm nothing was missing.

Fantastic feature, but you need to refresh every single page once to get the button to appear.

Cool add-on. I noticed it doesn't work on live videos (doesn't show the new button and the shortcut key doesn't work). Is this a known limitation or could it be something related with my specific configuration? Also, if I use the old youtube interface, the top bar (with the YT logo, search bar, etc.) overlaps the full-screen videos, but I doubt that this will affect many users as it's getting harder to disable the material design UI/polymer framework. Thanks for the add-on!