Earlier quoted context omitted.
Unfortunately Firefox doesn't have a good UI/UX after all. The last time I checked, multiple profiles support is somehow half-baked.
Multiple profiles is fully supported. It's just hidden behind a clunky UX. Also check out firefox containers which is to profiles what docker is to virtual machines.
firefox --ProfileManager
And then to use them you have to start firefox e.g. : firefox -P
Very few casual users (nor even most technical users) start Firefox from a command line, and setting up shortcuts for these is also a step that most users won't do.The support for profiles is there, it's just hard to use in the context of a GUI desktop.