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 used to used a *dactyl plugin (can’t remember which one exactly, I remember there being a couple) for FF before Mozilla switched to the new plugin framework and it was fantastic. Since then I’ve switched to Vimium. Is there anything to be gained from tridactyl over vimium?