Multiple instances of an application on the same X server?

Question

I'd like to run multiple instances of firefox in lxc containers, using ssh X forwarding:

ssh -X container-IP
firefox

It works perfectly if this copy of firefox is the only one. If I previously start a firefox on the host system, or in another container, then I get a copy of the first in a new window. How can I run them parallel on the same X server? Is it possible?

(Both the host and the containers are systemd based... Ubuntu 17.04 and Fedora 22 - I don't know if it is important or not)

Answer 1

O.K., I wasn't too patient... The solution is, start firefox with the '--new-instance' option. The "research" was simple: I started firefox with --help.