So, not uBlock or whatever as everyone’s aware of that.
Mine (all Firefox):
Video Speed Controller. LOVE this one. A tiny widget appears on any video showing current speed and you hit a key to +10% / -10% (I have mine mapped to [ and ]). You can also hide the little widget (v) if you’re watching a movie etc.
Very handy for sites that don’t support changing video speed or have limited options.
Quick cookie manager - Allows you to delete all cookies on a page that you’ve visited. Very handy when “reject all” leaves you with like 20 cookies anyway.
Netflux - 1080p Netflix on Linux. Can be a little fiddly at times but it does work.
For Firefox:
Auto Tab Discard: makes living with a hundred open tabs manageable.
Copy Selected Tabs to Clipboard: allows copying the tab title + address in various formats like Markdown. (There’s also an addon like this for links, I forget what it’s called.)
Imgur redirect blocker, Load Reddit Images Directly: block the webpage wrapping for images on Imgur and Reddit
Leechblock NG: blocks specified sites after a certain time spent on them. Useful for Reddit and such.
Lemmy Keyboard Navigation: a sliver of the functionality of Reddit Enhancement Suite
Save In: to quickly drop an image or a file download into a particular directory under the downloads folder
Search By Image: self-explanatory
Swift Selection Search: when text is selected, shows a popup to search it on various sites. Useful to invoke Wikipedia, Wiktionary, IMDb, etc.
Temporary Containers: like a private window, but in tabs instead, and with multiple private sessions at once. Plus the tabs survive browser restart.
Viewhance: proper image viewer in the browser, with zooming, rotating, etc.
Stylus and ViolentMonkey: to patch looks and behavior on sites that I use often.