I'm on Vivaldi so I don't know how many of these are available to Firefox. Leaving out all the obvious ones like adblocker, password manager, userscripts, etc.

Privacy Pass; do less captchas. Every time you solve a captcha, it stores a few "tokens" in your browser, essentially verifying you as human extra times at once. The next few times you encounter the same brand of catcha, your browser will "spend" one of those tokens to automatically be treated as high confidence, skipping the captcha.

Bot Sentinel; puts a little score next to people's names on Twitter, showing how often they've been reported to the Bot Sentinel site for various things like spam, trolling, or hatespeech; it's nice to know at a glance when you just shouldn't engage with someone.

Jiffy Reader; when it's enabled, hilights the first couple letters of every word, which is great for ADHD because it makes your automatic reflex be to look at each word one at a time, rather than skim the whole section.

Teleparty; watch netflix, etc, with friends, with a little built-in chatroom

Trim; show IMDB/Rotten Tomato ratings on netflix, etc, thumbnails; a real minor tweak, but I'm a big fan

Beyond20 and the VTT Enhancement Suite; specialized D&D addons that made playing online so much easier during the pandemic. Beyond20 pipes your character sheet macro rolls from D&D Beyond directly into Roll20, and VTTES adds all sorts of bonus functionality to Roll20.