It was working like a charm, but at least for me, it stopped working.

Cause a lot of issues in pausing and unpausing video, which has already been raised on github.
I suppect it has something to do with Enhancer for Youtube, but I cannot really provide the details.

Did not work IF window is minimize/inactive.

Worked fine for a long time and I already forgot that this annoyance exists.

Recently it broke Media Keys however (both about:config settings TRUE). When not in focus in some burried tab, sometimes with another application in focus, play/pause quickly reverts making the play/pause media button useless. Clicking on the Win10 overlay which then pops up also quickly reverts back to the play/pause state it had before. Works when I'm on the affected youtube tab again and breaks easily when it's a burried tab again. Was looking everywhere on why the media button broke for my win10/firefox recently, but it's fixed with this addon deactivated.