Yes, very. This is not specific to Firefox, but anything running as root gets access to everything. Only one thing has to go wrong for the whole system to get busted.
usually logged into KDE Plasma as root.
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.