Firefox has an old slightly broken feature that wipes all cookies, except from a set of origins you whitelist. It actually saved me about half a gigabyte of pure cookie nonsense and made website loading quite a bit faster. Soon after I set it up, they announced their third-party cookie sandboxing, but I still think there's no reason to keep all the adtech trash on your computer in any capacity.

about:preferences -> Privacy & Security -> Cookies and Site Data -> [x] Delete cookies and site data when Firefox is closed -> Manage Exceptions

Just don't forget to back up your `~/.mozilla/firefox/*.default-release/cookies.sqlite*` beforehand.