If you have firewire devices connected and they are recognized by your Mac, System Profiler should be able to list the devices.
If it does not, I would try logging out and logging back into a different account, and trying System Profiler again.
If you get the same result (no devices shown), try zapping PRAM (hold down Command, Option, P and R keys simultaneously when booting the Mac, and keep holding them until you hear 3 or more startup chimes).