Firefox doesn't seem able to connect to a specific open profile and make a new window, if you run Firefox without --no-remote, it always opens a new window for the oldest instance, which makes keeping multiple profiles open a pain. If you close your browsing profile while your have your email profile still open, now the Firefox icon will open windows in your email profile.

If you make an icon for your various profiles that uses '--no-remote', you can only open one window with that icon. To open more you'll have to find the running instance.

I generally prefer Firefox so if I've missed something I'd love to know, but I searched high and low for a solution and in the end I gave up and swapped to Chrome. Now I have launchers for Facebook, etc. that behave exactly as you would expect. You click the Facebook icon and you get a Facebook window, running the Facebook profile.