Here's a list of those I have installed:

• uBlock Origin https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/
• Consent-O-Matic https://addons.mozilla.org/en-US/firefox/addon/consent-o-matic/
• SponsorBlock https://addons.mozilla.org/en-US/firefox/addon/sponsorblock/
• Youtube-shorts block https://addons.mozilla.org/en-US/firefox/addon/youtube-shorts-block/
• ClearURLs https://addons.mozilla.org/en-US/firefox/addon/clearurls/ (no longer needed since last update afaik)
• Proton Pass https://addons.mozilla.org/en-US/firefox/addon/proton-pass/ (or bitwarden if you use a password manager)
• Bypass Paywalls Clean https://gitlab.com/magnolia1234/bypass-paywalls-firefox-clean
• LibRedirect https://addons.mozilla.org/en-US/firefox/addon/libredirect/
• Firefox Relay https://addons.mozilla.org/en-US/firefox/addon/private-relay/
• SteamDB https://addons.mozilla.org/en-US/firefox/addon/steam-database/

Some that haven't been mentioned yet:

• Behind The Overlay (removes simple login banners/paywalls/anything that blocks the content)
• Don't Fuck With Paste (for those sites that think it's fun to override or even disable copy and/or paste)
• ~~Greasemonkey/Tampermonkey~~ Violentmonkey

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!<

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.

Dark reader https://addons.mozilla.org/en-US/firefox/addon/darkreader

Absolute essentials:

• Ublock (ad blocker and tracker blocker). There's some ssential filters for it as well, but that's a separate story, I guess)
• Consent-o-matic: (Autofills cookie prompts, by answering no to everything)
• Multi Account Containers (separates login info, useful when you have several logins for a single site)
• DarkReader (dark mode for webpages)
• Bitwarden (password manager)
• ClearURL (strips tracking elements from URLs)

For Youtube in particular:

• Sponsorblock
• Return youtube dislike
• Youtube nonstop
• Any auto hd plugin (haven't found a "best one" yet)

For Reddit in particular

• Reddit RES (lots of qol)
• Old Reddit Redirect (good for when browsing Reddit when not logged in, e.g. in private mode)

Ublock origin

Any extension to re-enable right click. It’s annoying when they try to block me from downloading a picture/video, copy pasting, or inspecting elements

LibRedirect for me, works well on both desktop and android

recipefilter.

Makes most recipes appear as a modal dialog covering the stupid blogspam that the sites put up.

There's another one that I can't remember the name of on my desktop computer that allows you to block domains from web searches. Like pinterest.

Edit: it's uBlacklist

There are only *two must have extensions in Firefox:

1. uBlock Origin
2. Your password manager’s browser extension

Beyond that it’s all optional. Most things I used to use extensions to accomplish are now possible to accomplish using Firefox’s built in settings or using uBlock Origin.

There are a few other extensions I use that I consider useful but optional:

1. Dark Reader
2. Facebook Container
3. Libredirect

Must have:

ublock origin.

Optional:

Cookie auto delete.

Nitter redirect.

Sponsor block.

Redirector

Add custom URL redirects, e.g. automatically use Piped or Invideous instance instead of YouTube, use Nitter instead of Twitter, remove Google amp

Edit: I just found out about Libredirect from this post - seems like functionally the same thing I'm using Redirector for, but with rules built in.

Don't have Firefox but the first plugin you must install on any browser or turn in your browsing license is uBlock Origin.

uBlock Origin.

I really love the built-in container extension (Multi-Account Containers or something like that). Really good if you need to log in to the same site multiple times or if you don't want someone track you across sites.

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.

uBlockOrigin, DarkReader & ProtonPass

• DownThemAll!

• Firefox Translations

• Imagus

• uBlock Origin

• Grammarly

Dark Reader
uBlock Origin

Optional:
TWP Translate (is better than the new feature)
Tabliss (Another take on the "new page" in chrome)

Ublock Origin or AdGuard Containers

Nobody mentioned Tridactyl yet so... Tridactyl. It's the best vim keybindings extension for Firefox I've tried.

uBlock Origin

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

Beyond the other obvious choices of DarkReader, uBlock Origin, and Tree Style Tab:

• Leechblock NG - limit time spent on addictive websites

• Tab session manager - save tabs in a more flexible way than the default behaviour

• Augmented Steam - Price history etc on Steam, including third party stores

I recommend:

1. Ublock
2. Sponsor Block for Youtube
3. Return Youtube Dislike
4. Hide Shorts for Youtube (I actually like shorts, but only on my phone)
5. Stylus
6. Decentraleyes
7. Don't Accept image/webp
8. Rotate Image (rotates images in 90 degree increments)

uBlock Origin

KeePassXC-Browser (Or your preferred password manager)

Consent-O-Matic

LibRedirect

Bypass Paywalls Clean

Flagfox if you wanna see where a site is hosted. Provides little actual benefit but it's cool to have anyway!

All the ones that are most important to me have already been mentioned.

• Ublock origin
• Cookie delete
• Bypass Paywalls Clean
• Firefox Relay
• Dark Reader

Must have is quite personal tho

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.

uBlock Origin and some kind of mouse gesture with rocker commands extension. Those are the only two universal types I use. Everything else I can't live with is pretty specific to my own usage to alter the function of specific websites (like RES for Reddit, but for other sites).