Library Extension: Set it up with your local library and it will tell you if a title you are looking up on Amazon is available in your local library or not. No signups. Only negative is that it’s not open source.
Privacy Badger: Stops trackers from tracking you all over the internet.
Privacy Redirect: Redirects Youtube, Twitter, Instagram and Google maps to Invidious, Nitter, Bibliogram and OpenStreetMaps respectively. Invidious is unreliable and Bibliogram keeps getting blocked my Instagram but works if you change instances. I have resorted to using mpv+youtube-dl for youtube. So. much. better.
uBlock Origin: DIE ADs DIE! Also useful to block annoying UI elements on the web.