@april's XKit Rewritten and Palettes for Tumblr: A variety of optimizations and changes for Tumblr.
Shinigami Eyes: Flags users and organizations and the like as transfriendly and transphobic.
Decentraleyes: Blocks most outbound requests to networks like Google Hosted Libraries, using local files to avoid breaking sites. Complements other content blockers.
HTTPS Everywhere: The EFF's extension to force HTTPS encryption all over the web.
Bitwarden: A free, secure password manager. Also has a desktop app.
Privacy Badger: The EFF's tracker blocker.
UBlock Origin: Efficient, customizable adblocker. Works on YouTube.
Bypass Paywalls Clean: Bypasses news site and some academic paywalls.
Auto Tab Discard: Automatically sleeps unused tabs (does not actually close tabs, don't worry.)
Wayback Machine: The Internet Archive's archiving extension- also allows you to pull archived versions of pages.
Search Filter: Can eliminate sites like Pinterest from search results.
Simple Tab Groups: Allows you to group tabs, hide tabs, and switch groups easily. Is difficult to learn, but good once you get the hang of it.
Multi-Account Containers: Allows you to contain accounts within different container tabs, letting you be logged into multiple accounts on the same service in the same Firefox session.