I have been able to run snow leopard by executing
rm -rf /System/Library/Extensions/*Firewire*
I think need to sneak this somewhere into the yosemite installer..
Or perhaps install yosemite with a different laptop and then swap the drive into my old firewirebroken laptop after running my above command.
The yosemite upgrade hangs around the halfway point..
I can't justify buying a new logic board for this machine since I don't have a need for firewire. Just want to hack the OS into functionality.