Not a huge fan of the new Firefox Proton UI, the tabs look more like buttons and I found the previous design a lot more streamlined. The new design doesn't feel as usable as it should be IMO, and it'll be a reason for me to move to another browser if the design isn't updated in the future.

At the moment, I've switched off Proton in the about:config section, but it's only a matter of time before this will no longer be an option. Hopefully Firefox will go back to the old UI or at least make some changes!