kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows

The Firefox Profile Manager allows you to create an additional profile while retaining your original one. This is useful not only for having separate settings for multiple users, but also for troubleshooting.

support.mozilla.org/en-US/kb/profile-manager-create-and-remo...

Firefox stores your personal information and settings in a profile folder. Learn to use the Profile Manager to work with different profiles.