Firefox

Yup, on Firefox, profiles are just directories of settings, and you can have as many as you choose. AFAIK, few people use them, since you can get most of that behavior with container tabs. For example, at work, I have one container group for work Gmail, one for personal, and if my wife uses it, she can open another group for her stuff.

I'd use profiles for a shared login on a computer, but I'd just use separate user accounts instead. I use container tabs for multiple logins for the same service on one profile. Switching then just means opening a new tab with that container group.