This works fine, but the problem is that it is not near as layman friendly as Chrome's "Switch User" feature that is available a click away on every Chrome window. Only tech aware people will fiddle with command-line switches and create shortcuts, yet even an average user would like to use multiple simultaneous profiles/users.
Firefox needs to improve on Chrome here. The erstwhile "Profile Switcher" addon provided a nice UI but is sadly gone after the WebExtensions transition and I cannot find an equivalent so far.
http://kb.mozillazine.org/Shortcut_to_a_specific_profile
Also don't forget to add -no-remote option to each of the links to enable starting such browsers in parallel. Sadly it's not mentioned behind the URL above.