The worst thing about the Proton UI that I never hear anyone talk about is that it is objectively a massive downgrade if you're not using a modern 1080p (or higher res) monitor. My laptop is an old X220 Thinkpad, so that change actually affected my ability to work efficiently by pointlessly wasting a ton of screen space, and it's part of the same trend in software engineering of user-hostile decisions that impact people who either can't afford or don't want to use newer hardware for whatever reason. Except in this case it's not even something like making everything a bloated Electron or React app, it was just changing the UI to something that looks way worse for no reason other than to create the superficial impression that Mozilla still cares about Firefox. I run a customized userchrome because Proton sucks so much for me to use.