Firefox introduced new profile manager which can be enabled via this config:
browser.profiles.enabled
Official doc: Manage Firefox profiles | Firefox Help
It is not compatible with old profile management system.
But I could not found:
- any command line arguments to create or open new profile,
- documentation about which directory stores the profile data on file-system (this is important for me)
Its really interesting that they don’t document this kind of information. very suitable for the business world 
Source ? it seems primarily an UI revamp
Looking at related articles in the article you linked would have given you the answer, see Profiles - Where Firefox stores your bookmarks, passwords and other user data | Firefox Help
Profiles which created via new profile manager is not visible to old profile manager (and visa versa). Anyone can test this simply in 30 seconds locally.
Random testers from internet, writes the same thing: How to Enable the New Profile Manager in Firefox 138
Old profiles won’t appear in the new interface, and vice versa.
This is for old (existing) profile manager.
I tested it with 140 nightly, and see profiles are stored in same location, as in old one (138 without new profile manager). But it’s a snap version in ubuntu
/home/“user”/snap/firefox/common/.mozilla/firefox/
There is option to add old profile to new profile manager. Just create new profile, rename it (e.g. to .old) and rename old profile to newly created. Similar is in the opposite direction.
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