JohnHorb

In that case, just set the trusted interface to (say) 192.168.10.1 and make it the DHCP server for the LAN. You can either leave the router as DHCP server JUST for the firewall external interface, or give the firewall external interface a static IP of (say) 192.168.1.2 (if the router is 192.168.1.1).

Your client PCs will then all get 192.168.10.x addresses.

Alternatively, if you have any static IPs on the LAN, switch everything round so the LAN remains on 192.168.1.x and give the router and firewall external interfaces 192.168.10.x addresses.

(All this assumes you are using a subnet mask of 255.255.255.0)