Earlier quoted context omitted.
Multi-Account Containers are like profiles, but in tabs instead of other windows. They're pretty great. It's an official Mozilla Firefox extension: https://addons.mozilla.org/en-US/firefox/addon/multi-account...
If I understand correctly, these only compartmentalise cookies and browsing history, whereas full profiles (via the -ProfileManager switch in Firefox) compartmentalise the whole browser config (add-ons, permissions, etc.) as well, which I think is much more useful.
I use distinct profiles for testing, but during the day, I open versions of the site for personal and work for example. Or at work for different clients.
It depends on workflow IMO. There are uses for both.