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.

To me it's the opposite. I'd like the same browser config, use the same window, keep my bookmarks, history etc., just want different logins and website contexts. I love Firefox's implementation of containers, and find Chrome's way of doing profiles useless.

But Firefox supports profiles as well if that's what you want, as you say. So best of both worlds.

If you want to open 5 of the same website but with different accounts, containers are the way to go.

The AWS console comes to mind.

I don't want separate configs, just separate accounts.

I have also used it as a proxy manager. Different proxy config in a different container.

Both have their merits, in my opinion.

Given that Firefox already has multi-profile support internally, it would only be a matter of adding a GUI similar to Chrome's to make it the best of both worlds.

Yes, containers compartmentalise the web-facing side of the browser, not the user-facing side.

This fits with my use case for profiles.

Depends on usage.

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.

Yes, perhaps I should have said 'more powerful'. Utility is definitely relative to usage.