So, not uBlock or whatever as everyone’s aware of that.
Mine (all Firefox):
Video Speed Controller. LOVE this one. A tiny widget appears on any video showing current speed and you hit a key to +10% / -10% (I have mine mapped to [ and ]). You can also hide the little widget (v) if you’re watching a movie etc.
Very handy for sites that don’t support changing video speed or have limited options.
Quick cookie manager - Allows you to delete all cookies on a page that you’ve visited. Very handy when “reject all” leaves you with like 20 cookies anyway.
Netflux - 1080p Netflix on Linux. Can be a little fiddly at times but it does work.
Maybe not that unknown, but:
Vimium - lets you control the browser by keyboard. It uses vim keys for navigation, but I think it’s useful for non vim users too because it also has a mode that adds text ‘hints’ for all the links on the page so you can click them by typing (I’m not describing it well so just check it out)