I had no idea you could do that via about:config 😅 I would always either disable "Use the selected profile ..." or launch it with firefox -P --no-remote as suggested.
FYI after years of having several profiles for different activities, I settled down on Multi-account containers and have been quite happy with it. I even wrote a little blogpost about my experience sometime ago - you might find it useful.
It doesn't work the same way as Chrome, as a warning though. If you open links from other applications, it will only open in the first instance of Firefox.