You can make OS links (e.g. different shortcuts on the desktop to different profiles) to start with different -p [profilename] then starting different profile is just a click away. I do it all the time using such configurations.
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.
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.