There is an easier way, if you are willing to install the Profile Switcher for Firefox extension and accompanying connector. See my other comment: https://news.ycombinator.com/item?id=28356472

You can create a shortcut which will launch with the profile of choice: https://support.mozilla.org/bm/questions/1171685

The problem is that this is the same solution as it was in 2005 and no average user is mucking about with specified shortcuts or command line flags. Chromium has demonstrated the most obvious GUI for this but for whatever reason the FF product team believes we should prefer about:profiles or this solution.

I launch my FF profiles from the terminal using aliases. But I guess some people would find that difficult (though it's not) even if they do happen to be a Linux user.

I just have a shortcut for launching with the -P (Profile) and -new-process options.