Please don’t do this! DEs are not tested to be run as root! Millions of lines of code are expected to not have access to anything they shouldn’t have and as such might be built to fail quietly if accessing something they shouldn’t in the first place. Same thing applies to Firefox, really.
Could you elaborate on this? I’m genuinely surprised because Fedora just asks you if you want to have the option to log into root from KDE during installation, so I always just assumed that it’s intended to be used that way.