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 almost destroyed a phone once when I scrolled down to an unexpected spider image, my fight or flight response kicked in, and I threw the phone across the room. So this sounds amazing!!
If only mobile Firefox had even a little bit of extension support.