Firefox

SponsorBlock. An absolute necessity if you watch youtube on desktop. It skips host-read sponsors in videos, as well as other stuff you might want to skip like intro animations and Interaction Reminders ("don't forget to like and subscribe!").

Usability

• Kill Sticky: Kill off the annoying floating things blocking the website you're trying to see.
• Tranquility Reader: Like native "reader view" but compatible with other addons and more options.
• Scroll Zoom: Zoom web pages with the left or right mouse button and the scroll wheel.

Image / Video

• Image Max URL: Finds larger/original versions of images (supporting 8800+ websites), including a powerful image popup feature
• Invert Image: The add-on inverts color of an image or color of any part of a page. Changes white color to black, for comfortable night time reading.
• Save webP as PNG or JPEG: Convert any image (WebP, AVIF, etc.) to PNG or JPEG (with choice of quality) for downloading.
• TinEye Reverse Image Search: Click on any image on the web to search for it on TinEye.
• Video Speed Controller: Speed up, slow down, advance and rewind any HTML5 video with quick shortcuts.
• Enhancer for YouTube™: Take control of YouTube and boost your user experience!

Tools

• EPUBReader: Read ePub files right in Firefox. No additional software needed!
• WebStickies: (Persistent) Sticky notes for the Internet

RSS

• RSSHub Radar: RSSHub Radar is a spin-off of RSSHub that helps you quickly discover and subscribe to RSS and RSSHub for your current site.
• RSSPreview: Preview RSS feeds in-browser

Customization

• Stylus: Redesign your favorite websites with Stylus, an actively developed and community driven userstyles manager.
• Tampermonkey: Tampermonkey is the world's most popular userscript manager.

Advanced

• Request Control: An extension for controlling requests. See also Redirector, not as powerful, but much more user friendly.
• Modify Header Value (HTTP Headers): Add, modify or remove a header for any request on desired domains. I use this one to force sites to load only the image when opening images in new tabs.
• Cookie AutoDelete: Control your cookies! This WebExtension is inspired by Self Destructing Cookies. When a tab closes, any cookies not being used are automatically deleted. Keep the ones you trust (forever/until restart) while deleting the rest. Containers Supported
• uBlock Origin: Finally, an efficient wide-spectrum content blocker. Easy on CPU and memory.
• uMatrix: [EDIT-WARNING: as pointed by @sovietknuckles@hexbear.net, uMatrix it's not longer maintained since 2021] Point & click to forbid/allow any class of requests made by your browser. Use it to block scripts, iframes, ads, facebook, etc.

Besides the one super-obvious (uBlockOrigin), my favorite single one would be Tab Center Reborn, which together with the styling from Firefox Vertical Tabs pretty accurate recreates the superb vertical tabs of MS Edge.

And on a desktop screen, I can't imagine going back to horizontal tabs that waste the previous vertical space I got.

I really like consent-o-matic on firefox. You can set your cookie level to (dis)allow, and it goes through them automatically when you land on a site.

There is "i dont care about cookies", but I do care, I dont want your cookies and I dont want to go through your dark patterns!

clickbait remover for YouTube.. it replaces thumbnails with an actual frame from the video

Ublock for sure it's the best and only add-on i use

ublock obviously should be installed on Firefox by default. But I seem to have a host of privacy add-ons that break few-to-no websites.

• Privacy Possum , which blocks certain tracking headers/js. Privacy Badger by the EFF is an acceptable alternative but I've personally found it doesn't block quite as much.
• NoScript Honestly my favourite addon of all time. You can operate in block-everything mode and just allow javascript/HTML5 from sites you trust, or if you're lazy then just operate in allow-everything mode and every now and then set crummy sites to untrusted (looking at you google tag manager). In block-everything-by-default mode, this add-on will break some sites, but the UI is so easy it's a couple of clicks to trust all the sites in a tab and auto-refresh.

Be warned - If you're not privacy conscious, you might cry from seeing the hundreds of sites that are running javascript on your machine without asking.

• User-Agent Switcher Really easy add-on to just leave on and misdirect sites. Never caused me a single problem, and in fact is useful when sites (looking at you Microsoft Teams) claim they don't work in Firefox and refuse to load but actually work fine if you use this addon and pretend to be Chrome.
• Sponsorblock kicks ass. 30 hours of ads skipped in half a year.

And my personal silly couple ones:

• Wikipedia Vector Skin because I'm an old fuddy duddy and I like old Wikipedia.
• Cat-In-Tab because I'm also an old fuddy-duddy that likes whimsy sometimes. This is just silly but I like it.

These are the ones I cannot live without/use everyday:

• Bitwarden (I self host a vaultwarden instance)
• Facebook Container
• Firefox Multi-Account Containers (Makes using services like Office365 with multiple accounts at the same time much easier)
• Private Internet Access
• Return YouTube Dislike
• Redirect AMP to HTML

I have a few others installed that have already been mentioned plenty of times like SponsorBlock, uBlockOrigin. Not using an ad filter these days is like fucking a stranger without a condom, you're just asking for super syphilis.

• uBlock Origin: On medium mode. Honestly, the internet mostly sucks without this excellent extension.
• Dark Reader: Easy on your eyes and prolongs battery life on OLED displays.
• Redirector: This allows you to be in full control of which sites/urls you redirect and to where. As it allows the use of regex, you're even able to create your own 'bangs'. For example I used !x as a bang to redirect me to my favorite SearXNG instance. Kinda neat.

Aside from the usual security & blocking, there is

• Behind The Overlay Revival
• Chameleon
• Color Changer
• Image extract
• JPEG XL viewer
• PassLok Image Steganography wonder where I've used that?
• Redirect AMP to HTML
• Save Screenshot
• SVG Export

Aye, another saved post in my Lemmy account.

Blocktube. Simple and advanced blocking of videos on youtube. Worth getting. Also stops youtube from auto pasuing after having watched a while.

Consent-o-matic. Set your preferences and it will automatically click those specific preferences when you visit a website, where it recognizes the cookie accept popup. No more need to click accept all, to move on. (It does not know all types of cookie pop ups, but it knoes the ones used by a large chunk of websites)

Imagus. Mouse over an image and this extension will attempt to show it (on your mouse location) in larger size. This works great for various things. News articles, social media etc. It can even do it for video and gifs. It can be annoying in some cases but.. ive gotten used to it. I used to use it on reddit. Just mouse over the posts titles and the pic would pop up. And as said before, super nice for articles and photo albums etc

Sideberry, its like Tree Style Tabs but IMO is much more configurable and refined. It's honestly changed the way I use browsers, being able to bookmark entire trees of tabs, toggle between tab sets, and manually load/unload trees and groups. I legitimately worry about the extension api changing and disallowing it.

Cannot say whether someone has already said it or not, but Flagfox. I like to know if possible whether or not a site is hosted in certain countries or just might be curious sometimes as to where the site server is located.

tree style tabs / sidebery. it's life changing to be able to organize your tabs and actually read the titles.

• uBlock origin of course
• Dark Reader
• Open in Reader View, which allows me to open a link directly in reader view. This can actually bypass some login walls surprisingly enough.
• Activate Reader View, which allows me to force a website to render in reader view even if the browser decides not to show the icon in the address bar.
• LocalCDN which hosts some CDN resources locally (it's a more frequently updated fork of Decentraleyes)
• Open With, which allows me to open a url in another browser or another command. I mainly use it for feeding urls to yt-dlp.
• Web Archives, which allows me to open a link in Archive.org among other archive services.
• LibRedirect, which allows me to open links in privacy frontends (up-to-date fork of Privacy Redirect)

For mobile only,

• OldLander, which makes old.reddit.com more usable in mobile. I started having issues with almost every teddit instance once the API changes came in, so I decided to bypass the frontends and use reddit directly.

ublock origin

A few of my favourites.

• DeArrow
• Dark Reader
• SponsorBlock
• Identify

I've been using IPvFoo on all my PCs since I wrote it for Chrome 12 years ago. Recently I made it fully Firefox compatible. It's useful if you have IPv6 and want to see which websites are on board, though it's a bit depressing if your ISP only offers IPv4.

I've found it particularly interesting on Lemmy, because it connects to such a wide variety of independent servers:

Clickbait Remover for Youtube makes that site a lot more pleasant. It'll replace all thumbnails with a screenshot from early in the video, and un-capitalize all titles to make them consistent.

I don't think I've seen this mentioned, likely because it's a simple, non-privacy extension.

Reading List. It works like bookmarks, but it is targeted at news articles and other things you want to read but don't necessarily want to "forever save".

I just realized i have no FF extensions active. Ive just been rawdogging the internet this whole time.

adaptive-tab-bar-colour

• uBlock Origin (medium mode)
• Dark Reader
• Video Speed Controller

A couple that aren't already mentioned by others:

• FeedBro RSS Reader
• Don't Track Me Google - prevents google from replacing its search results and other links with redirect URLs
• Vimium - Vim style keybinds to navigate pages

Tree Style Tabs is a must for me. It makes the tab/tab goup functionality in other browsers feel awful.