Works perfectly fine with Waterfox too, using 2.0.124.2 on Waterfox G6.0.14 (64-bit).
Manifest needed a tiny edit :)

your extension saved my soul from youtube's awful new UI - THANK YOU!

This WAS an awesome add-on, but not anymore. It ends up making your videos not load, or STOP video, while audio continues. If you're having SERIOUS ISSUES watching YouTube and you have this installed, REMOVE IT!!!

its perfect!!

A must have extension, unless you chat in livestreams, in which case it forces the chat position under the video, makes no sense, has been broken since most recent update