I see some of these have already been mentioned, but they do deserve repeating;

• µBlock Origin - blocks ads, and does it well.
• 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.