Ok, considering the AHT (Apple Hardware Test) showed no issues
This could be what is called a "phantom FW device issue".
This can be as the result of a 3rd party device which could affect the performance of the firewire ports on your PM G5's logic board.
Now if you are comfortable with going inside a short ways into your Power Mac,
Here are some steps below to reset the PMU on the logic board, to see if this can
aid in removing the "unknown device" message you see in System Profiler
• Pull ALL cables (pwr, USB, etc) and let sit for 10 minutes; reconnect & restart.
• Press the SMU reset button (ONCE) on the logic board; it is halfway between the lowest RAM slot and the bottom edge of the logic board.
• Reset the PRAM:
4) Open Firmware reset for PPC:
After the startup tone, hold down Apple Command key, Option, O & F keys, until the text screen is visible. When text can be entered, type the following commands, followed by the Return key after each line:
Reset-all (the Mac will boot up after this command is entered)
Your Mac should restart and your FireWire ports may be available.
If that does not work, try turning off your Mac and unplug from the main electricity
for 15 mins to reset the FireWire ports.
See if this helps the Firewire unknown device issue.
I have a strong feeling, that as long as the unknown device message is not affecting the performance of any of your Firewire 400/800 ports when connecting devices,
this issue may be just that, a "phantom FW device issue".
Let me know if this helps.
I will keep watching for your updates.