I'm pretty sure the jb3 is only a usb 1 device, so it won't really matter what you have on your computer, it will only transfer as fast as the jb3 will push the data to the pc.
Firewire cards are CHEAP -- should be able to find one for $20 --
Regardless, the jb3 is passing the music to the pc as data, firewire and usb SHOULD be equally accurate in the transfer, just a lot longer on usb (but still faster than realtime transfers)