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!
Haven’t been able to live without it for at least 15 years now. I remember years ago the switch to the ‘new’ addon system when Firefox had a major update. I stayed on the old Firefox until Tree Style Tab was ported to the new one.