Work well with firefox 51.0.0
Didn't work in my FireFox 34.
It works with Firefox 26.0, also with Youtube Center. But there should be an on-off switch.
it doesn't work in my firefox 25.0.1. scroll and video window goes out of view
Please feel free to contact me on Support E-Mail mentioned on extension's AMO page. I need to understand why it is not working for you.