Kbase Article of the Week: Power Mac G5 (Late 2005), iMac G5 (Ambient Light Sensor): FireWire Bus Stops Responding After Computer Restarts with FireWire Audio Device Attached →

Apple Support:

Issue or symptom
If a FireWire audio device is connected to a Power Mac G5 (Late 2005) or an iMac G5 (Ambient Light Sensor) while the computer is restarting, the FireWire bus will stop responding and no FireWire devices will be detected on the bus. The device may stop responding or stop charging, depending on the specifics of the device.

You can restore the Firewire bus by resetting the SMU (System Management Unit).