Firefox

• Stylus - I use it to apply some css to make youtube thumbnails smaller because the new huge ones drive me insane
• Plasma Integration - Pretty self explanatory, just makes firefox better integrated with plasma
• SponsorBlock
• UltraWideo - useful for weird content with baked in letterboxing or pillaring

Chameleon. My user agent changes every 30 seconds. Makes attempts to track me basically useless.

The usual +

• Stylus with a global dark user style

I think it's more performant and lightweight than dark reader. So I get more screentime.

Highlight or Hide Search Engine Results

i still use google, so being able to filter out the blogspam and fandom slop is a godsend for usability

Gesturefy: It brings the mouse gesture functionality known from Opera to Firefox.

uBlock Origin and DarkReader

Recently got into using RSS feeds, so RSSHub Radar was quite useful
Search by Image is good too

• Ublock
• Dark Reader
• Libredirect
• augmented steam
• proton pass

uBlock Origin (this is the real one) uBlacklist (this blocks sites from your search results)

For me, it's many of the ones people have already said, plus:

• StreetPass (seriously cool - collects the mastodon profile of any website you visit where someone has set up the special link to their profile)
• Video Speed Controller (gives you fine-grained control over video speed, e.g. watching video at 2.6x speed)
• Privacy redirect (automatically redirects to various services, e.g. from Twitter to Nitter - can select a random instance each time)

• uBlock Origin
• Dark Reader
• Indie Wiki Buddy
• Reverse Image Search
• LibRedirect

Must haves IMHO:

uBlock Origin Consent-o-Matic

Making life easier:

SponsorBlock Enhancer for YouTube DarkReader Multi Account Containers

Tab Snooze - allows you to close a tab and have it reappear at a chosen time later

Media URL Timestamper - automatically inserts the current timestamp of the YouTube/Twitch video you're watching and updates it in the history in case you accidentally close/navigate away from the page or go to a different time in the video

Feedbro - RSS reader with filtering capabilities

Redirector - auto-redirect specific URLs (for example, changing a YouTube Shorts url into a regular one, or changing Reddit links to always go to Old Reddit)

Undo Close Tab Button - allows you to restore recently closed tabs including the tab's history in the back button (max amount = browser.sessionstore.max_tabs_undo)

Violentmonkey - using userscripts that allow you to change things on websites.

• For Instagram - unmuting videos + setting their default volume
• For YouTube - disabling the subtitles/captions + disabling "autoplay next" for playlists + disabling autoplay of channels homepage video
  • a way to disable specific keyboard shortcuts (you need to manually add the code as a new script). I use /^(Key)?(End|I|O)|(Digit|Numpad)\d$/ instead of /^(?:Digit|Numpad)\d$/ (thanks to this post), to also disable the End/I/O keys in addition to the number keys.
• Note that, at least for Violentmonkey, if the userscript doesn't have the "://" part of the url in the @match line then you need to add it in the userscript settings after installing the script (for example, if the @match line of the script only has *.youtube.com/* then put *://*.youtube.com/* in the "@match rules" line in the settings)

YouTube Comment Reader - allows you to search through the comments of a video (by clicking on the addon in the Extension menu and then clicking on the "YouTube Comment Reader" at the top or the "X Comments" at the bottom of the tooltip)

Page Shadow - allows you to use dark and light themes on sites that don't have the option to change it.

And if you're like me and you find that some YT videos feel too slow but 1.25x is too fast, then you can use Enhancer for YouTube's "Playback speed" feature to have smaller speed steps. Then you can hold ctrl and use the scrollwheel (while over the video) to change the video's speed by the amount you chose (I use 0.05 speed variation, mostly changing to 1.05x or 1.10x)

I use these ones frequently:

• Quick Currency Converter - I'm interested in a better currency converter, its UI is not nice, but otherwise usable, and I couldn't find a better one
• Open Page in Private Window - Adds button to rightclick menu
• Want My RSS - RSS button in the url bar
• Viewhance - better image viewer, zoom with scroll wheel, etc
• TWP - Translate Web Pages - Like the built in translator, but online only, works with more languages, translate selection in rightclick menu
• PersistentPin - I don't know why FF forgets pinned pages, but this solves that

Ublock Origin, NoScript, Chameleon, Libredirect, DarkReader, OneTab, Stack Overflow Prettifier, Classic Mode For Wikipedia, Vimium

• uBlock
• Privacy Badger
• Language tools (spell checker)
• Mal-Sync (automatically updates Anime & Manga progress with MAL, AniList, Kitsu etc. Supports Netflix, Prime and some high seas website)

Fakespot for amazon/ebay reviews

Dark Reader: Especially late at night white page background just burns out my retinas, no idea how I ever managed before.

Consent-o-matic !