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!
One on Firefox that’s super helpful for work is Simple Tab Groups. It lets me keep plenty of tabs open without it turning into a disorganized mess. Better than Chrome’s built-in tab organization, imo.
I recently discovered STG, and I cannot stress enough how great it is! It takes a minute to set up with colors and group names. But, oh my, how great it is to have my Spotify, Last.fm, Bandcamp, etc, in a Music tab, all my work stuff in a Work group, all my general stuff (Facebook, Lemmy, and so on) in a Home group, and so on. Being able to toggle between them is so useful.