In general, it's better to limit the number of addons/extensions as each one is another risk (e.g. if the developer goes rogue)... also recommend to use several browsers with different purposes/addons (e.g. Brave just for searching for stuff, with no cookies/history... Firefox where you just visit sites regularly and login to places).
To be used in both Firefox/Cliqz and Chrome/Brave/Vivaldi:
The only adblocker you should use (unless you have one built into the browser, like Brave does)
Intelligently stops tracking between sites. Not an ad blocker, so can be used in conjunction with one.
Ensures your connection to the site you are visiting is encrypted
Bitwarden (or other password manager)
An addon for my choice of password manager. It generally better not to rely on the one built into your browser, as you are locking yourself into them and can be a nightmare to export/import them all later...
N.B: Some say you shouldn't store passwords anywhere near a browser (better to just copy/paste them from an offline encrypted database) as they are inherently insecure environments... but this is too far.
I don't care about cookies
Blocks those annoying popups asking about cookies. It just accepts them.
(this is a feature built into 'Cliqz' browser so no need if you use this browser)
Delete cookies, unless its a site you care about staying logged into - in which case you whitelist them (works with Firefox containers too!)
Useful for research. If you find an article behind a paywall, this might help find another source.
History Cleaner / History AutoDelete
I won't ever need to remember which websites I visited more than a week ago, so may as well clear the history.
Useful for when you need to give an email address... these services generate a site specific email that forwards to one of your actual email addresses
Behind the Overlay Revival
A kind of popup blocker.
Strips URLs (web addresses) of all the crap typically used to track you
Joplin Web Clipper / Turtl
Addons to clip websites to these encrypted notebooks applications, if you have them installed. Joplin is better, as it actually takes the entire website to your notes, so you can read them offline or add your notes directly to them and dont have to worry about sites going offline. Turtl just bookmarks the site with a nice icon.
Prevents those annoying 'click all the traffic light' verification tests.
Terms of Service; Didn't Read
Doesn't actually do anything. But tells you how shady the TOS of the website you are on is.
Hard to explain what this does... but recommended by many.
Also interesting, but not essential:
TrackMeNot / AdNauseam
These 2 go together well. The first conducts random searches on your computer (obscuring your actual searches) and the second silently clicks on the ads (reducing the value of ads). Better to just use uBlock Origin, but you might have a use for this if you don't have a VPN and want to pollute your data.
Flowcrypt / Mailvelope
Two very easy ways to encrypt your webmail with PGP. But your friends also need to use PGP... better just to use ProtonMail / Tutanota...
Allows you to send encrypted messages to people on Twitter/Facebook etc if they are also using Keybase.
You can make the internet think you are using a different browser or computer (to reduce the amount of info websites have on you)
Allows you to easily switch your internet traffic through a proxy (e.g. Tor or JAP network). Useful if you don't use a VPN or the Tor browser
Ghostery / Duckduckgo
Similiar to uBlock Origin and Privacy Badger respectively.
It allows your browser to contribute to the Tor project
Useful if you use many differet browsers for different purposes. I think is Mac only.
Allows you to save articles you read, for reading later. Built into Firefox and endorsed by Mozilla, so probably 'OK' as far as privacy concerns, but would still use a fake generated email to register with them.
Firefox Multi-Account Containers / Facebook Container
Makes containers for websites, to prevent you being tracked by them across tabs (e.g. you signed into Facebook ONLY it its container, so the 'LIKE' buttons you see everywhere won't link back to your facebook account. This is under-rated.
A very easy way to make temporary email addresses.
When you load a YouTube video, it automatically takes you to a privacy-centric mirror of YouTube, called Invidious.
Mendeley / Send to kindle
Good for research/reading, but bad for privacy so maybe have in own browsers dedicated for research.