uBO, of course. note: you guys don't need ClearURLs with this list added.
LibRedirect for automatically opening Youtube, Twitter, TikTok etc. links in their privacy-focused front-ends. I just make sure to disable all the instances by esmailelbob since he's a little homophobic shithead
Buster for automatic captcha solving
Consent-O-Matic automatically clicks through cookies banner to deny all the cookies that aren't necessary, which I like better than just hiding the cookie banner
Redirect AMP to HTML because fuck AMP and fuck Google

Firefox user here.

• Bitwarden password manager
• Bypass Paywalls Clean
• Clear URLs remove URL trackers
• Highlight or Hide Search Engine Results to hide some unwanted websites from search results
• Open in VLC™ media player, useful for some weird streams
• Push to Kindle sends any text article to PDF or to your ereader (not only Kindle)
• Recipe Filter filters recipe pages on blogs and just gets the actual ingredients & instructions
• Redirector for a few paywalls where I use a specific proxy
• RSS Reader Extension (by Inoreader) - as I use Inoreader for following RSS feeds
• Sci Hub Injector adds sci-hub links to many science publishing websites for easy access
• Shinigami Eyes highlights trans-friendly and transphobic social media users or websites
• uBlock Origin
• ViolentMonkey for userscripts

Extensions to be helpful to other people:

• Picket Line Notifier tells you if the website you are visiting has workers on strike - useful especially for ecommerce & news publishers
• Snowflake is not noticeable for me, but allows other people to use my network as a Tor node or something idk
• Wayback Machine archives every page I visit on the Internet Archive.

Fediverse extensions:

• FediAct allows me to boost, reply to, follow, etc. on any Mastodon instance without having to open the right link in my own instance. I wish there was something like this for Lemmy and Peertube.
• Fedishare allows for one-click sharing to several Fediverse platforms, including Lemmy and Mastodon
• PeerTubeify tries to check if a YouTube video you're watching is also on PeerTube

Youtube extensions:

• Auto HD / 4k / 8k pour YouTube™ - I use it for the environment, so default quality is 480px (because usually I watch the videos on a small side window so it doesn't change the visible quality)
• Clickbait Remover for YouTube - replaces thumbnails with a frame from the video and makes all titles normally named, no all caps
• DF YouTube (Distraction Free) - removes the homepage & sidebar on videos to avoid rabbit holes
• SponsorBlock auto-skips sponsored segments, intros, credit rolls, etc. on YouTube videos

I have created a FOSS extension called SyncMarks to sync bookmarks and tabs. It's working with Firefox, Edge and Chromium and also on Kiwi on Android. You can sync your bookmarks independent from the browser and cross-browser. For example from Firefox to Edge or Chrome.

As a backend I would recommend my small php script which you can selhost. You only need PHP and a database like SQLite or MySQL. As fallback you can use any WebDAV share.

FoxyProxy in addition to many aforementioned extensions. Tor and popular VPNs just don't work in my whereabouts, so, I have to use something more sophisticated like shadowsocks, for example, in order to circumvent government censorship and geoblocks.

One I haven't seen mentioned yet is DownThemAll.

DownThemAll lets you download all the links or images on a website and much more: you can refine your downloads by fully customizable filters to get only what you really want.

Comes in really handy sometimes. (For Firefox / Chrome / Edge)

Another is uBlacklist, which allows you to blacklist domains from Google / Bing / DDG search results (like say, pinterest.*), also for Firefox / Chrome / Edge.

One that I love is jumpcutter. Speeds up silences and makes watching long lectures way nice.

If I get back to my PC I'll send a few more extensions I use.

uBlock Origin and Sponsorblock

uBlock Origin - as ad and script blocker

Dark Reader - for dark mode on every site

Sidebery - for tab management

For privacy & security:

• DuckDuckGo Privacy Essentials
• uBlock Origin
• Privacy Badger

For usability:

• Wappalyzer - Technology profiler
• Firefox Translations
• Flagfox
• Grammar & Spell Checker—LanguageTool

In addition to all good recommendations above, I also can recommend Vim Vixen or Vimium C.

These extensions enable you to control your browser with your keyboard with vim-like commands. If you are already using vim or want to use your keyboard more to comfortably navigate your system, it is a must-have :)

Firefox:

• uBlock Origin (uBO) - The internet is basically unusable without this. {GPLv3}
• Dark Reader - I like using dark themes and I hate when I get blasted with a light theme when I visit a site. This keeps that to a minimum. {MIT}
• Firefox Multi-Account Containers - It's nice to keep things separated. {Mozilla Public License Version 2.0}
• Consent-O-Matic - Automatically marks my saved cookie preferences on consent pop ups. This is a great tool to help counter to the dark patterns related to GDPR, but it isn't perfect. {MIT}
• NoScript - I don't like giving blanket permission to run JavaScript in my browser. This let's me choose. {GPLv3}
• Wayback Machine by Internet Archive - Archives the sites I visit automatically and provides a one click option to visit an archived version of a URL that returns 404. {Proprietary | I don't know of any alternatives}
• Tampermonkey - There are a few very useful scripts that I run periodically. Tampermonkey keeps them organized and easy to run. {Proprietary | I don't know of any alternatives}
• Reddit Enhancement Suite - I got a lot of value from this extension over the years, but I don't know how much value it has going forward for me {GPLv3}

On LibreWolf and Brave, I use:

• uBlock Origin
• Privacy Badger
• BitWarden
• Decentraleyes
• Redirector
• Facebook Container (LW only)
• FoxyProxy (LW only; not FOSS)
• AnonAddy
• Load Progress Bar (LW only)
• Xtreme Download Manager
• SponsorBlock
• Return YouTube Dislike (redundant, since I use CloudTube, but good to have nonetheless)
• Facebook Container (LW only)
• Ruffle
• ClearURLs
• FlagFox (LW only)

Simple Tab Groups for Firefox, I couldn't imagine using the internet without it. A bunch of Lemmy tabs filling up your tab bar and crowding out your normal tabs? Just make a new group and slap them over there, now all your other tabs are hidden in the other group and you can switch between them anytime.

I had one class where I needed like 10 tabs open all the time, I could just have them sitting in a different group so they didn't take up any space for my other tabs.