Thanks for your work but it does not work at all for me. Seems like my watched Videos are not (all) in my history! Only Videos I watched on my mobile device are in there. Maybe AdBlocker for YouTube prevents this?

After newly moving to Firefox, it took me some time until I noticed the button in the adress bar when on YouTube, so a bit of a hint would have been good.

Apart from that, it does what "Hide Youtube Watched Videos" does on Chromium browsers