IME, starting Firefox with this flag, then (from the profile manager) picking an already-running profile, I get the fatal error:

> Firefox is already running, but is not responding. To use Firefox, you must first close the existing Firefox process, restart your device, or use a different profile.

So I guess it allows multiple instances but only with different profiles, at least on my system.

If that's true (I haven't tested), then --no-remote is what you're looking for.