BTW, since nobody expanded on it, outside of the usual UI complaints Firefox 91 did bring a number of very nice improvements. They include:
*
HTTPS first.
Until now, if you typed in the URL bar or clicked a bookmark for 'overclockers.co.uk', Firefox would initially attempt to visit the domain with plain http. You would then rely on the web server at the other end to either have HSTS implemented to upgrade the connection to HTTPS, or else an extension in the user's installation to do similar (HTTPS Everywhere, HTTPS-only mode). This leaves you vulnerable to MITM attacks, with a malicious actor intercepting the plaintext exchange and serving you their own certificate in reply. With Firefox's new HTTPS first policy,
any typed URL or bookmark will always try to connect over HTTPS/TLS first, and only then downgrade to plain http if no encryption is available. Nice.
* Cookie improvements with '
enhanced cookie cleaning'.
Following on from the non-global 'cookie jar per domain, not per user' improvements of recent versions of Firefox. With Firefox 91, deleting or forgetting a site from History now removes all cookies, service workers, cache etc from
all domains embedded in that site, not just the site itself. Previously, deleting/forgetting website.com would only remove the cookies and other data for website.com. Now it deletes the data for website.com, plus facebook.com, twitter.com, doubleclick.net and any other domains embedded in/used by that website. In other, simpler words, the cookie jars are now stored 'per website' rather than strictly 'per domain'. This cuts down even more on potential cross-site tracking vectors.
* Faster paints. Users will see 10-20% faster UI interactions throughout Firefox.
Don't forget to switch the Enhanced Tracking Protection (Settings > Privacy & Security) to Strict for these and more benefits.