Hi Swamy,
firewalls usually should never be opened for services you don't use
In contrast to ports like TCP port 80 for HTTP it's very unlikely that you find a firewall which is opened for incoming RMI connections by default. Furthermore you're of course free to choose any port for it if you have administration privileges. Perhaps you'll even need two ports for an RMI application because there are the RMI connections themselves and connections for the RMI lookup like rmiregistry.
Marco