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Wayback Machine by Internet Archive - Archives the sites I visit automatically and provides a one click option to visit an archived version of a URL that returns 404.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}