Using SuSEfirewall for the internal lan

Hi I am getting 1 of probes to port 80 since yesterday to my home network which consists of SuSE 7.1 firewall/router serving 3 1 which are also running SuSE 7.1 and 1 Windows 98 laptop. Although firewall/router has ipchains blocking for all ports except forwarding port 25 can using ipchains firewall in the masquareded 5 increase my security and if so how will it affect the performance. I am planning to use Marc's SuSEfirewall-4.6 but I could not figure out what is my dev_world if I just say eth0 then it will get the address of the NIC but that is not how the packets are coming to the 0 lan since it is masquared. Any guidance is 0 TIA -- Togan Muftuoglu