"Firefox is already running" with -no-remote

I would like to open a new instance of Firefox. I tried

firefox -no-remote

and

MOZ_NO_REMOTE=1 firefox

as detailed in the docs. However I keep getting the message

Firefox is already running, but is not responding. To open a new window, 
you must first close the existing Firefox process, or restart your system.

How can I launch a new instance of Firefox from command line?

Answer

The -no-remote option is used for running multiple Firefox profiles at the same time. You are getting the "Firefox is already running" message because you are trying to open a second Firefox instance with the same profile.

You should be using this option as follows:

firefox -no-remote -P "Another Profile"

or

firefox -no-remote -profile "profile_path"

where profile_path is an absolute or relative (to firefox.exe) path.