Your music processing happens on the PC, not the firewire drive. The data is stored on the firewire drive, but moved to the PC for processing by the music application.
Firewire is more independent of the PC processor and does not depend on it, or bog it down so much. Less possible glitches.
Firewire is preferable to USB with hard drives for all applications. That's why it is used for video. For everyday use, USB will work. It is used because it is provided by PC manufacturers (it is cheaper). I always have a firewire port on my PCs and laptops.