I'm a long time user of FF.

I really don't like the UI changes in this upgrade, but setting:

"browser.proton.enabled" to false

"browser.uidensity" to 1

(while using the `System Theme`)

Seems to have done the trick, so, I can just ignore the UI changes (which is great)... fingers crossed in that this keeps working in the future.

UI changes are very much subjective, so, I think that it's great that some people like the new UI better and keep using it as long as there's also a way to not alienate existing users that like the existing UI.

Settings probably will be removed as soon as possible. Take a look at UI customizations that can be made using userChrome.css : https://www.userchrome.org/firefox-89-styling-proton-ui.html