Alternatively, if you want to flip back to the old UI in Firefox 89, you can go to about:config, search for browser.proton.enabled, set it to false, and then restart. That should do it too.
Huh, on Arch GNOME it does not go back to the old interface, but it does make the new one more compact for me. That is far more workable for me.
For how long, until they remove it? Honestly, I hate Firefox now. It's going backwards.
I think you can safely assume they won't throw it away instantly. Unless they introduce some new feature that is only possible with the new UI, Mozilla will probably just keep it around to appease fans until they redesign it again.
No, they are confirmed throwing it away in the next release.
Hopefully after the next esr comes out so I don't have to deal with this for another year