Incidentally, one of the biggest changes is the containment of so-called 'supercookies':
In Firefox 85, we’re introducing a fundamental change in the browser’s network architecture to make all of our users safer: we now partition network connections and caches by the website being visited. Trackers can abuse caches to create supercookies and can use connection identifiers to track users. But by isolating caches and network connections to the website they were created on, we make them useless for cross-site tracking.
Security researchers of the University of Illinois at Chicago have discovered a new method to track Internet users that is persistent across sessions, even if users clear cookies and the browsing cache. The research paper "Tales of FAVICONS and Caches: Persistent Tracking in Modern Browsers" highlights that favicons may be used in conjunction with fingerprinting techniques to track users.
Scratch that last quote -- it seems an unreported bug has made Firefox immune to this attack: