Usability

• Kill Sticky: Kill off the annoying floating things blocking the website you're trying to see.
• Tranquility Reader: Like native "reader view" but compatible with other addons and more options.
• Scroll Zoom: Zoom web pages with the left or right mouse button and the scroll wheel.

Image / Video

• Image Max URL: Finds larger/original versions of images (supporting 8800+ websites), including a powerful image popup feature
• Invert Image: The add-on inverts color of an image or color of any part of a page. Changes white color to black, for comfortable night time reading.
• Save webP as PNG or JPEG: Convert any image (WebP, AVIF, etc.) to PNG or JPEG (with choice of quality) for downloading.
• TinEye Reverse Image Search: Click on any image on the web to search for it on TinEye.
• Video Speed Controller: Speed up, slow down, advance and rewind any HTML5 video with quick shortcuts.
• Enhancer for YouTube™: Take control of YouTube and boost your user experience!

Tools

• EPUBReader: Read ePub files right in Firefox. No additional software needed!
• WebStickies: (Persistent) Sticky notes for the Internet

RSS

• RSSHub Radar: RSSHub Radar is a spin-off of RSSHub that helps you quickly discover and subscribe to RSS and RSSHub for your current site.
• RSSPreview: Preview RSS feeds in-browser

Customization

• Stylus: Redesign your favorite websites with Stylus, an actively developed and community driven userstyles manager.
• Tampermonkey: Tampermonkey is the world's most popular userscript manager.

Advanced

• Request Control: An extension for controlling requests. See also Redirector, not as powerful, but much more user friendly.
• Modify Header Value (HTTP Headers): Add, modify or remove a header for any request on desired domains. I use this one to force sites to load only the image when opening images in new tabs.
• Cookie AutoDelete: Control your cookies! This WebExtension is inspired by Self Destructing Cookies. When a tab closes, any cookies not being used are automatically deleted. Keep the ones you trust (forever/until restart) while deleting the rest. Containers Supported
• uBlock Origin: Finally, an efficient wide-spectrum content blocker. Easy on CPU and memory.
• uMatrix: [EDIT-WARNING: as pointed by @sovietknuckles@hexbear.net, uMatrix it's not longer maintained since 2021] Point & click to forbid/allow any class of requests made by your browser. Use it to block scripts, iframes, ads, facebook, etc.