https://support.mozilla.org/en-US/kb/profile-manager-create-...
Also if you want to prompt what profile to use every time you use firefox just add -P like this "firefox.exe -P" in your shortcut/softlink or script that starts firefox.
The very basic requirement for me is this: I want to completely separate my "work" internet from my "personal" internet, yet I also want to be able to easily switch between the two. Firefox makes this unnecessarily difficult and un-ergonomic.
What's the most frustrating is Firefox already has multiple profile support. All it's missing is an ergonomic way to switch and manage the profiles.
Too bad it is just an extension and not a first-class feature. The settings are not synced with other Firefox settings. Somewhat annoying as there is lot of work to setup the container settings if you happen loose then.