All the usual suspects named previously: uBlock Origin, Bitwarden, ViolentMonkey, Vimium, etc. Plus:

* Buster Captcha Solver --Poetic justice in browser extension form! Uses Google's speech recognition software against Google's ReCaptcha to solve the captchas, This browser extension more than any other has probably saved me from going on a killing rampage against whoever invented ReCaptcha

https://github.com/dessant/buster

* Typio Form Recovery --Fantastic extension that allows you to recover the data you've just typed into a browser web form and which has been lost through browser crashing / page reloading / etc. Has saved me tearing what's left of my hair out on many occasions:

https://typiorecovery.github.io/

* Shift Click --As the name suggests. Shift+Click on any image on a webpage and it will open the image in a new tab. Great for quickly getting the full-size version of images on web pages and more useful than it sounds, as it saves opening the browsers dev tools pane and source code diving. I use it all the time on eBay to inspect full size images of items I'm looking at and which eBay will only display at a certain size:

https://shft.cl

* Country Flag & Whois --not exactly a necessity, but a fun extension anyway. Gives you a wee flag at the side of your address bar, showing you which country the server of the site you're visiting is hosted in. On a more practical note, you can click the flag to view the Whois info for that domain:

https://add0n.com/country-flags.html

* Asccidoctor.js Live Preview --A while back I started using AsciiDoc for all my text composing needs, after finding the limitations and ever-multiplying variants of MarkDown increasingly annoying. Unfortunately, while many text editors come with Markdown preview capabilities, I've yet to find one that can do AsciiDoc previews. So that's where this extension comes in. I open my AscciDoc files in my browser and this extension automatically generates a preview, which I can then 'print to PDF' to have a nicely formatted document created using AsciiDoc:

https://github.com/asciidoctor/asciidoctor-browser-extension

* Stylebot --Inject custom CSS into any webpage. I use it on a couple of sites to fix annoying browser rendering quirks or remove irritating content [with display:none;].

https://stylebot.dev

* FireNvim --a nice accompaniment to Vimium. This extension turns text entry windows in web forms into NeoVim edit bufffers, allowing you to edit text using an embedded NeoVim instance. I don't actually use this as much as I thought I might, as I nearly always forget I have it installed:

https://github.com/glacambre/firenvim

These last two aren't browser extensions as such. But I can't imagine using my browser without these. So I think they deserve an honourable mention:

* Pinboard bookmarklet --I use Pinboard to save all my bookmarks. So they're accessible across all my browsers and all my devices. I have the Pinboard bookmarklet on the browser address bar on my desktop machines [and use the 'Share' menu on Android] to quickly bookmark pages from wherever I'm browsing.

* Telegram --One step down from permanent bookmarking via Pinboard, I use Telegram as my messenger app and avail of this facility all the time on Android to quickly stash links I want to take a look at later on a 'proper' computer --either to view on a larger screen, or because [as with this post] I want to compose a reply, without wrestling with a crappy touchscreen keyboard. Telegram has a 'Saved Messages' chat. So, on an Android device, I hit the 'Share' icon beside the URL on a page I want to save for later and then in the Share window that pops up, I send the URL to Telegram 'Saved Messages' where I can grab it later, when I'm on a real computer.

For anyone using Yandex Mail; Yandex also have a Telegram Bot which provides a similar function but also emails you the link as well. I used to use this til I realised the emails where pretty superfluous and just started sending links to my Telegram 'Saved Messages' instead:

https://t.me/YandexMail360_bot

EDIT: A couple more I found in my browser's preferences but which I forgot about because you don't directly interact with them:

* Redirect AMP to HTML --as the name suggests, tells servers you want to be served up the HTML version of a page rather than the AMP version because... fuck Google, basically.

https://www.daniel.priv.no/web-extensions/amp2html.html

* Nimbus Screenshot --Comes in handy sometimes when you want to grab a screenshot of part of a webpage which is too big to fit in the browser window without scrolling. Nimbus Screenshot has various capturing modes including 'Select and Scroll' which allows you to do this.

https://chrome.google.com/webstore/detail/nimbus-screenshot-...

* Chrome Extension Source Viewer --Allows you to download Chrome extensions as CRX files from the Chrome Webstore. Thus allowing you to manually install them on browsers which refuse to install them directly. I've used this to successfully install a couple of Chrome extensions on Yandex Browser in Android, which the browser refused to install, telling me they were incompatible.

https://github.com/Rob--W/crxviewer