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!
Based on a quick look, the biggest features it has that browser bookmark managers generally don’t have is the ability to search within saved webpages and documents without opening them. Plus, you can share bookmark collections with other people. Sounds a bit like a modern rendition of del.icio.us