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!
The <T> shortcut alone makes Vimium a must, it makes switching between tabs so much easier. The only drawbacks of the extension I’ve found are having to adjust settings for the odd websites that have shortcuts and certain elements not working well with Vimium “clicks” (like the Lemmy sort order dropdown list!).
And it borks the YouTube shortcut keys, but I still love it. Incredibly useful when using a laptop/track pad.