Ublock Origin is an obvious one, but I also can’t stand not having Foxy Gestures anymore. It adds customizable mouse gestures, so you can set it up to have easy swipes to go back a page, reload a page, close a tab, etc, and it feels wonderful and smooth to use compared to just using the traditional buttons to do everything. Honestly it’s kinda wild to me that this isn’t more popular now that people are so used to phone gestures. It’s good for the same reasons!
I wish you’d recommended that commenters keep to “one extension per top-level comment,” making it easier to upvote the best ones 😅
Anyway, my “one pick” is definitely Tridactyl — a thorough, absurdly powerful poweruser-mode for FireFox based in Vi-like modal interaction.
https://github.com/tridactyl/tridactyl