exactly. i cant believe nobody here seems to know about profiles or about:profiles. its the perfect solution. my setup is tree-style tabs (recently made to work with 57) and a different profile for each task (one profile for each language im learning, one for music, and one for each research topic i might be pursuing). whats your setup like?
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.
But containers don't have separate extensions, bookmarks and history so they can't completely replace profiles.