Small Business router to firewall configuration advice

Howdy,

I am working on setting up a small business with a Watchguard firewall behind their Comcast modem. I had Comcast give us static ip's but my boss and I argue about how to setup the modem. I believe the Comcast modem should be setup in passover mode (dhcp & nat turned off), and he wants to place the firewall in a DMZ and set the external interface of the firewall to the modem's internal ip range.

*The office will use VPN's and a BOVPN connection with their firewall and not much else.

Any advice would be greatly appreciated.

Thanks!