I’ve also been experiencing the joys of FireWire and Win 8.1 and been scouring the web for solutions.
You will all pleased to know that MS have released a signed Legacy FireWire driver!
http://blogs.msdn.com/b/usbcoreblog/archive/2014/09/10/announcing-the-availability-of-a-standalone-legacy-1394-ohci-firewire-package.aspx
Try to share this on any boards you’re on as there’s so much talk about using the Win 7 drivers that it took a surprising amount of time to find this!
Enjoy! 