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!
This is has been so useful in my attempts to go FireFox full time. I always had Edge and Firefox on the work laptop to separate things when needed, but the containers completely eliminated that.
My one critique would be if the assigned links wouldn’t open an empty tab when that specific container opens.