What make / model is this firewall you're talking about mike2002 ?
This is also HIGHLY dependent what exactly you want to do. If you only have one cable coming off the ISP-supplied
equipment and/or want a single chain of devices from the ISP-supplied equipment, then you could configure a
secondary ip address on the 1841 as follows :
ip address x.x.x.x [subnet mask here]
ip address y.y.y.y [subnet mask here] secondary
If the ISP-supplied equipment allowed it, or you didn't mind the setup, you could connect a layer2 switch
and have the 1841 take the /29 subnet while your firewall did the /25.