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.

ublock origin

Aside from the popular ublock origin I’d get Sponsor Block. Too bad my mobile version doesn’t support it.