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 i done see mentioned is OneTab. Allows me to take large groups of tabs and dump them to a text file, which I can also use to restore groups of tabs. It allows me to close entire browsers full of tabs but keep the links to prevent firefox from getting out of hand with ram consumption.
You can do something similar without any addons. Firefox allows selection of multiple tabs at once out of the box, and you can have it create bookmarks for this selection. You can then have it open all bookmarks in a bookmark folder at once.
Yeah I love this feature and use it all the time. Then I never look at those bookmarks again lol