On Firefox:

• uBlock Origin
• Bitwarden
• Decentraleyes
• Terms of Service; Didn’t Read
• Firefox Multi-Account Containers
• Facebook Container
• Consent-O-Matic
• Don’t Track Me Google

…and the add-ons not related to privacy:

• 10ten Japanese Reader (Rikaichamp)
• Zhongwen
• New Xkit
• Earth View from Google Earth
• Palette Creator
• Save to Notion
• Send to Outline
• Web to EPUB

I use most of these on Brave Browser, too.

• Adguard (Adblocker)
• ProWritingAid (Grammar Checker)
• Volume Master (Boosts tab audio)
• Chrome Remote Desktop (Obvious)
• Honey & Rakuten (Coupons and Cashback)

I also use a Chromebook, so I use these “native” extensions I developed.

• CrosKeys (Launcher, Clipboard Manager, Scripting, basically a Rofi replacement)
• CrosPaper (Wallpaper/Screensaver Manager)
• CrosManager (System Manager)

• AdNauseam (for cases when other extensions miss something)
• Bypass Paywalls
• Decentraleyes
• Don’t touch my tabs! (rel=noopener)
• Don’t track me Google
• Forget Me Not
• HTTPS Everywhere
• Old Reddit Redirect
• Redirector (to use the Vector theme on MediaWiki instances)
• uBlock Origin
• uMatrix
• User-Agent Switcher and Manager

I love these types of posts. From the outset they seem mundane, but they are discussion provoking and I’m able to learn of new things at the same time.

My list (Firefox) 🙃

• Absolute Enable Right Click & Copy
• Bitwarden
• Don’t Fuck With Paste
• I still don’t care about cookies
• SponsorBlock
• uBlock Origin

Some of you have a LOT of addons, jeez.

Edit: Updated to use the /still/ don’t care about cookies addon. Thanks @[email protected]

Looking at the comments, interest in privacy and adblocking seem high. But the actual addons suggested here are often redundant or even worse, make you easier to track. Instead, take a look at the Arkenfox wiki, which is more or less the gold standard for browser privacy. Both LibreWolf and Mullvad Browser, practically the most privacy/anonymity oriented browsers out there, right behind Tor, either rely on it or stick very closely to it.
I highly recommend reading the entire wiki from the start. You dont have to do everything it says, obviously, but even if you dont do anything it will give you a better understanding of browser privacy. And lastly, do not use Chrome or any chromium based browser if you care about privacy and a free and open internet. Even if you only use a chromium based browser, you help increase googles monopoly.

Firefox:
UBlock Origin
DuckDuckGo Privacy Essentials Plugin
Decentraleyes

Brave:
UBlock Origin
DuckDuckGo Privacy Essentials Plugin

I dont use HTTPS Everywhere anymore because its included in Firefox and Brave!

Edit: I only use Brave to do my school homework because firefox messes with the website we use sometimes.

More than I feel makes sense to post TBH. But a few important ones (for Firefox):

uBlock Origin
Sidebery to get my tabs in a vertical tree; if you often have more than 10-15 tabs I really recommend this!
Bitwarden (password manager)
Checker Plus for Gmail + Checker Plus for Google Calendar, easy-access email/calendar in a small window, I very rarely actually go to gmail.com
Return YouTube Dislike
Tampermonkey for userscripts on Kbin
Video Speed Controller, I use numpad +/- to increase/decrease speed by 0.25x, works basically everywhere including YouTube

Here’s my list. Using OperaGX.

• Bitwarden
• Buster
• Consent-O-Matic
• Dark Reader
• FastForward
• Picture-in-Picture
• Stylus
• TamperMonkey
• The Stream Detector
• uBlock Origin
• Session Buddy
• Return YouTube Dislike
• Looper for YouTube

• AdGuard Extra

• AdNauseam (uBlock Origin fork)

• Bitwarden

• Clickbait Remover for Youtube

• Custom Scrollbars

• Don’t accept image/webp

• Enhancer for Youtube

• Proton VPN

• RES

• Return Youtube Dislike

• Sponsorblock

• Thumbnail Rating Bar for Youtube

• Tree Style Tab

• Translate Web Pages

• uBlacklist (no more deviantart trash in image search)

• Redditsave (probably won’t need this anymore since i moved here)

• vidIQ Vision for Youtube (information on screen pleases me)

• Violentmonkey

• Youtube-shorts block

and thanks to the based commenters here, learned there were some userscripts for kbin and got the Dark Reader extension

• Bitwarden
• FoxyProxy
• DuckDuckGo
• AdBlocker Ultimate
• Privacy Badger
• LocalCDN

Here are the plugins I use:

• React Developer Tools (useful for developing with React)
• Ublock Origin
• Return Youtube Dislike
• SponsorBlock
• Grepper (shows convenient code snippets when I Google stuff)
• Cold Turkey Blocker (I use this to to block websites/apps when I need to focus)
• Shinigami Eyes (highlights queer friendly and anti queer subreddits/users/facebook pages/group differently)

On Firefox:

• uBlock Origin
• DuckDuckGo Privacy Essentials
• Privacy Badger
• SponsorBlock (skips the promotional parts of YouTube videos)
• Youtube shorts block
• 1Password
• Dark Reader
• Don’t Fuck With Paste
• Facebook Container
• Bypass Paywalls
• Disable HTML5 Autoplay
• Ghostery
• Old Reddit Redirect

I have these extensions pinned:

• uBlock Origin
• Firshot (takes screenshots of website with scrolling)
• Google Translate
• Wolfram Alpha
• Tampermonkey (let’s me write/add scripts to websites)
• Just a clock (I hide my windows startbar so having a little clock always displayed is nice)

And my remaining extensions are:

• Alternate Player for Twitch.tv
• BetterTTV
• Chrome Remote Desktop
• Confetti Snippets (adds copy button to code on Stack Overflow)
• DeArrow (Crowdsourced titles and thumbnails to remove clickbait from YouTube)
• Enhanced Steam
• Google Docs Offline
• HTTPS Everywhere (changes http links to https)
• JSONVue (formats and makes json collapsible)
• Lighthouse (tests website performance)
• Netflix Extended
• Reddit Enhancement Suite
• Refined Prime Video
• Resource Override (let’s me replace requested web resources with local versions of those files)
• Return YouTube Dislikes
• Save All Resources
• ShareX
• SponsorBlock
• SteamDB
• Tab to Window/Popup (adds keyboard shortcuts to between normal browser windows and the popup windows that don’t waste space with a title bar, useful for vertically split window layouts)
• uBlacklist (block sites from Google search results)
• View Image (returns view image option to Google Images)
• WAVE Evaluation Tool (tests website accessibility)

Bitwarden
Custom New Tab
Decentraleyes
OneTab
Privacy Badger
Proton VPN
uBlock Origin

I highly recommend checking out Firefox Multi-Account Containers.