In Windows it’s the same. Though the parameter is -P (uppercase) not -p. That’s why the comment said “it’s hidden behind a startup parameter”.

As best I can tell, there’s no way to make this into a shortcut that you could just click on.

I dont know about Mac, but in Linux you can just manually make a .desktop file to have as a shortcut to call firefox -P, or better a shortcut to a specific profile with firefox -P <profile>. Though what I often do is keep a bookmark to about:profiles and open a new window from there.

It was part of Firefox before Chrome was even a thing.

Many people aren’t aware of firefox -P and/or about:profiles… but it’s one of the oldest features in firefox.

Still, if they were actually being honest, this would be easily solved if they showed those “critical setup screens” BEFORE asking to create the account, and based on what the user selected then they would allow (or not) the creation of a local account…