Enter "about:profiles" into the URL field. The UI on this page is rough, but once you have more than one profile you will get a popup at launch to select from

Enter "about:profiles" into the URL field. The UI on this page is rough, but once you have more than one profile you will get a popup at launch to select from your list or to create more. I have dozens of profiles, custom launchers etc. I run four or five simultaneously most of the time. If you only need a couple profiles, the standard launch UI will do. ... But, see also "Multi-Account Containers" which segregate cookies between sites so you can have multiple simultaneous logins to various services in a single profile. Containers share preferences and browser config (and window), but segregate cookies. Profiles are completely independent. I use both, extensively. E.g. my work profile has containers for user, admin, owner, and machine accounts at GitHub. My personal profile has my private personal GitHub account only.