First it didn't work at all. Then I checked a link the dev posted as reaction to a 4-star-review and added "--disable-direct-composition-video-overlays" to firefox-command in terminal (using Linux). Works like a charm now. Thanks!

Edit: stopped working again after switching to another tab. hmm...
Edit2: Figured it out, it only works in actual fullscreen, not F11. Then even without the added line. Dumb me ^^