What are your favourite plugins that you think others are likely unaware of?

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.