Firefox now has vertical tabs. I for one use them (though in LibreWolf instead)

I’ve tried COSMIC DE Beta on Pop!_OS on an older laptop I have around, and the feature I’m looking forward to most, that I’ve seen and experienced from my mininal usage of it, is the ability to easily change colours from settings. Not just accent colours, but window colours, text colours, etc. From what I recall, I could, for instance, recreate Hot Dog Stand, the Windows 3.1 colour theme, right from the system settings app, no third party downloads