But does about:profiles work bug-free for you? From my experience it has had a long standing bug: when you select a different profile to the one you're currently using and click the "Open profile in new window" button, it actually loads the current profile again in another window. I read on a forum somewhere (forget the link) that this was an acknowledged problem and would be fixed sometime in future.

So at least until then, about:profiles is no replacement for using the -P command-line switch to start the profile manager.

Using multiple profiles seamlessly is one area Firefox can and should seriously improve, or Chrome's "User" feature will always have the edge.