I believe Firewire was developed by Apple and used primarily for video. It was adopted by Microsoft as the easiest way of getting video from a camcorder into Windows as the earlier camcorders only had a firewire interface probably as, at the time, USB 1 was the only USB standard that was available and it was too slow for streaming video.

With the advent of USB2 and USB3 camcorders became increasingly fitted with a USB interface and firewire is gradually being phased out. I think you will struggle to find an external HDD with a firewire interface and would advise USB anyway in view of the fact that its becoming a universal standard at the expense of firewire.

Forget firewire you won't see a drive with that sort of connection.

USB3 is fastest esata or USB2 common

for ease of use on an older PC USB2 is probably what you'll end up with, fast enough for back ups.

I leave my machine backing up and image overnight 200G tends to be a little slow over USB2 :0)