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.