Firefox doesn’t restrict how many profiles you run at the same time. You can start them either via -ProfileManager, or by going to about:profiles.

The --new-instance flag lets you run multiple copies of even the same profile.

https://wiki.mozilla.org/Firefox/CommandLineOptions#-new-ins...

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.