@kurtseifried @reinouts @kevinrns @ghorwood

It's about:profiles, not about:profile

Or you could make a shortcut that runs firefox.exe -P (capital P)

Once you have more than one profile the default when you launch Firefox is to ask you which one you want to use.

Or you could do what I did, ditch profiles and start using Containers instead, they're also built in and far more flexible, addons further enhance them.

Happy to help, have a nice day. :)