You can (now?) create profiles from the account icon in the toolbar [1] and at least on my firefox install, you can also do it from the hamburger menu.

I use firefox via flatpak and had no issues so far accessing profile data (in one of the folders in ~/.var/app/org.mozilla.firefox/.mozilla/firefox/ - I keep a regular archive of the entire folder as backup).

[1] https://support.mozilla.org/en-US/kb/profile-management