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!
Pushbullet - send stuff from my phone to my browser, or vice versa.
Camelizer - camelcamelcamel popup for Amazon browsing. CCC displays a price/time graph and lets you set alerts for when something is below a target price.
Youtube Playback Speed Control - I watch YT at 2x speed usually, sometimes 4x. This adds fine control and keyboard shortcuts for that.