Geb: Assign port to Firefox running instances

I am running tests in Jenkins CI written in Geb-Spock. There are multiple test project running parallelly and I want to assign different tcp port number to firefox instances from Geb configuration file or somewhere else.

As I investigated with netstat and found different firefox instances running on different TCP ports like 7055,7056,7057 and so on actually.

But I want to control it from within test specification.

Can someone share the KNOW HOW?