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Wayback Machine by Internet Archive - Archives the sites I visit automatically and provides a one click option to visit an archived version of a URL that returns 404.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}
I can second Firefox's containers. That has been my favorite feature Mozilla has implemented in Firefox and used with an add-on, of which there are a few. I've been using Containerise.
The Mozilla add-on is adequate and honestly probably better than Containerise in terms of UX. Containerise is simpler with fewer features, which could be a plus or a minus. I primarily use the add-on to automatically open a domain in a specified container and not much else, so it works for me.
For searching web archives, there's the Web Archives addon (GPLv3). It just opens a search page though, and doesn't have the ability to automatically archive.