Privacy Badger - blocks trackers, rewrites some tracking URLs, etc.
Multi-Account Containers - for those places where you want to keep tabs separate, giving each container its own cookies/session/etc.
Consent-O-Matic - automatically handles a lot of pages that shove annoying (and often technically GDPR-illegal due to lacking a quick "reject all" button) consent forms in your face.
Imagus - shows linked images on hover, including support for galleries and scrolling through all the images contained.
I see some of these have already been mentioned, but they do deserve repeating;
There was an imagus fork which is actively maintained