I can't be the only person who 1) totally doesn't understand why these exist (I strongly suspect they're a hangover from DOS-based MS Windows without proper user accounts), and 2) how to manage them (there are no profile management tools within Firefox that I'm aware).
Totally random and broken vestigial misfeature.
I've heard of others using this as a way to keep a slim browser but still have a swiss-army webdev browser for working with.
It's not the profile system that was broken it was some user settings that were dragged in through the upgrades from version 3 (when I last replaced the profile).
Other than admin accounts at work, it's been a few decades since I've shared an account with someone.
Even less to just click an alternate icon in the app menu.
You wouldn't need to do user switching per se anyway. You could just "su" with the other users ID. It's the things like fixing a printer adjustment twice. Getting a password so I can fix something or share a file. Fine in a supported env. but for home I'm happy sharing the same user (but not with the kids!).
It's just a command line argument.
Mostly, Firefox profiles mean I can't reliably generically script actions on my Firefox configuration (given there's some arbitrary random string involved).