For me firewire was too much cpu wasted on just keeping audio up.
Brand new machine 2010 - and could not get it down below 8%(20% one core).
And latency I could not get below 162 samples for WDM, or 192 for ASIO(Focusrite Pro 40).
But experimenting for three months this is how I got it working:
a) legacy windows drivers, was it 1094 or something firewire is called.
b) turn off in bios - core parking and dynamic overclocking.
c) see to that firewire cable is not bent but have really relaxed connection to interface
This is really the culprit as I see it - really old connector interface(from 90's or so).
Really long circuit board fingers, kind of, and a tiny bend on cable - and you are in for trouble.
d) cards I tested was TI and VIA - and they behaved ok both
Today I would go USB for external interfaces.