I use literally a dozen of them so I had to look at about:addons lol

• Basic JSON Formatter: Installed by default, quite useful to me as I deal with JSON API quite often.
• ClearURLs: remove tracking URL params
• Geminize: open Gemini links in a proxy
• HTTPS Everywhere: helps me redirect HTTP to HTTPS, but it breaks xkcd random button, sadly. I guess I should contact Randall.
• KeePassXC-Browser
• Native MathML: use MathML instead of the JS-rendered for math stuff
• Privacy Badger: Trackers blocking
• Privacy Redirect: redirect links to Reddit, Twitter, YouTube
• Read Aloud: TTS reader, because Firefox’s Reader mode TTS is somehow broken
• Redirector: For redirects that can’t be done with Privacy Redirect
• Stylus: Custom stylesheet, because some websites suck at styling
• uBlock Origin: beside trackers blocking, it also helps blocking sites I don’t want to visit, such as web scrapers or sites that are blocked by my ISP (e.g. medium blogs)