You have a Time Capsule and back up wirelessly with Time Machine, and that same Time Capsule is a server grade networked hard drive, so why do YOU need an external hard drive with firewire connectivity? The trend is "wireless", and with USB 2.0 the speed differences between USB/firewire are negligible for the majority of users.

EDIT: Did you get an Airport Extreme and not Time Capsule? Maybe you did? You can plug your external hard drive into the back of that Airport Extreme and use as a networked hard drive.