I cannot believe that a FireWire drive is limited to MacOS X 10.4 or above. FireWire is a standard that was developed by Apple. It should be independent of OS version as long as your hardware supports FireWire. By that I mean that you might have problems with FireWire 800 on a computer that supports only FireWire 400. However, your computer supports FireWire 800. Therefore, FireWire support should be a moot issue.
It does beg the question, however. Your computer supports MacOS X 10.5.8. If you need Classic, then you can run MacOS X 10.4.11. If you don't need Classic, then you may run MacOS X 10.5.8. So, why are you still running MacOS X 10.3.9?