Brilliant. DOES appear on Firefox android, it just auto-hides by default... It appears if you tap in the lower-right. This is obviously preferable to a permanent floating button that would be an eyesore.

The button doesn't appear on Android

Button doesn't appear on Firefox Mobile

This addon does exacly what I needed, and exactly what it says it will do. Full screen switching is pretty seamless with this addon, and it stays hidden (no annoying floating buttons to deal with). Only request would be to make the hidden button location, moveable. Good job dev!

It works however it gets in the way of many website/game buttons and becomes annoying. Button should be floating or movable. Even sizable.