> Admittedly, that should _really_ be exposed in the UI somehow
Go to about:profiles, click the "Launch profile in new browser" button under the profile you want.
Yes, it's not obvious, and the UI has a lot to be desired if you want to explain the feature to some non-techie, but you no longer need to close the other Firefox instances in order to launch a different profile. I think this was added a couple releases ago.