Yea, even tho it’s said completely broken on Firefox, it’s still works fine for me, but it had to be manually activated by clicking on channel page (or by clicking on next video) while you are currently watching the video. This will stay permanent unless I opened a video in new tab, which I had to do all of this again.
However on my 2 other PCs, the extensions still works out of the box without doing any steps that I did for my main gaming PC, same YT account, same browser versions (Waterfox G6.0.5), probably has to do with browser cache or something.