Firefox

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.

Ublock [1] and Noscript [2] are must have. you could also checkout Privacy Badger [3].

If you use arkenfox user.js [4] you could also use I still don't care about cookies [5]

• [1] https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/
• [2] https://addons.mozilla.org/en-US/firefox/addon/noscript/
• [3] https://addons.mozilla.org/en-US/firefox/addon/privacy-badger17/
• [4] https://github.com/arkenfox/user.js/
• [5] https://addons.mozilla.org/en-US/firefox/addon/istilldontcareaboutcookies

• Ublock origin: block ads
• Vimium: browse with vim like keystrokes
• Firenvim: edit text areas in neovim
• Dark reader: dark colors on webpages
• Containers: isolate browsing data

• Checker Plus for Gmail - Great way for me to see incoming emails in multiple inboxes, and mark as read/unread or respond
• Open With - quickly open a webpage in another browser
• SponsorBlock for YouTube - skips ads and sponsor segments in YouTube videos
• uBlock Origin - The best adblocker
• Video Speed Controller - Allows you to change the speed of a video, outside the normally allowed range (some slower videos are much better at 3x)
• Snap Links - Lets you draw a rectangle and opens all links inside. Great for checking out a list of links, like the links in this comment, for example