I know about the -P command, that is why I was referencing the -no-remote option ;) Firefox has multi-user, it's just janky.

You have to double click on your special shortcut with the command line option, and if some other program tries to open a url with firefox as the default browser with the -no-remote / -P command, then it will just not work if you have all users open.

While with chrome it's 2 clicks away and no edge cases making other apps opening urls not work.