Some that I use:

Dark Mode

I don’t like having a light screen.

• Dark Reader. This does a pretty technically-impressive-to-me job of making reasonable dark versions of pages. It’s not perfect – there are a handful of sites that it needs to be toggled off for, makes something hard to read – but I’m amazed that it does the job it does.
• Blank Dark Tab: Replace the new tab with a blank page matching Firefox’s built-in dark mode

Privacy/Anti-Tracking/Ad-blocking

• uBlock Origin. Ad blocker.
• Privacy Badger. Targets cross-site tracking, EFF project.
• Decentraleyes. Targets CDN tracking.

Paywalls

Some paywalls can be bypassed.

• Bypass Paywalls Clean

Tweaking Frameworks

• Stylus: Doesn’t do anything on its own, but permits collections of third-party themes to be applied to websites to fix annoyances.
• Greasemonkey. This doesn’t do anything on its own, but it permits people to publish little modifications to be applied to webpages, permits for a lot of little scripts that fix annoyances on websites. There were a number of useful scripts that I used on Reddit.

Misc

• Edit with Emacs. Permits opening the contents of a textarea in an external emacs instance. Nice for things like, say, writing a large lemmy post in Markdown. I vaguely recall that, at least some years back, there was a way to embed a version of vim in Firefox textareas, so if vim’s your cup of tea, that might be interesting, if it’s still around.
• Instance Assistant for Lemmy and Kbin. A variety of quality-of-life fixes for lemmy and kbin. Lets one open a given lemmy/kbin post on their local instance if they wind up viewing a page on a remote instance.
• Reddit Enhancement Suite. If you still use Reddit, this has an enormous collection of quality-of-life improvements for Reddit.

EDIT: I don’t know if this is the embedded vim that I recall, but Firenvim seems to do roughly the same thing, if not.

EDIT2: There’s also some “overlay remover” plugin that can bypass a number of obnoxious overlays that I use on my desktop, but I don’t have it installed on this machine. I think that it’s Behind the Overlay.

As a college student, my must have plugins are

• DarkReader
• Firefox Multi-Account Containers
• Sponsorblock
• TWP - Translate Web Pages

and the goat itself, >!uBlock Origin!<

Foxy Gestures

Surprised no one has mentioned VideoDownloadHelper yet - https://www.downloadhelper.net/

I’ve used this since I was a kid and it made me really popular in high school lol. Everyone thought I was a hacker or something.

Ublock origin Ghostery Containers (so darn useful) Tampermonkey if you know JavaScript, little tweaks can make some sites much more usable

Custom Context search (forgot the actual name of it and I’m on mobile now). It allows you to add custom searches to your right click, so if you select text and right click, you can search for it on any site with search functionality.

On desktop,

1. uBO, of course

2. Bitwarden (replace with your own password manager)

3. Redirect AMP to HTML

4. Redirector

5. Multi-Account Containers

With these five, you have control over the entire Internet. You can bend it to your will. On mobile, just 1, 3, and 4 (assuming you have your password manager installed at the system level, and until Containers works on mobile).

I also really like Notes for Firefox and Dark Reader, but they’re not what I’d call must-have.

• DownThemAll!

• Firefox Translations

• Imagus

• uBlock Origin

• Grammarly

uBlock Origin

Optional: Dark Reader Wappalyzer if you want to see cool web info

For me, I cannot go without the flagfox extension on PC. Otherwise, I’d probably just be going over extensions everyone else has been beating like a dead horse.

Since it hasn’t been mentioned yet, NoScript imo. Some sites run an absolutely absurd amount of scripts and the majority are not required for the site to function. So at best, there’s no value from letting them run.