Firewire is nice, but USB has become standard for the PC Market. In fact, wasn't that why they picked it for the iMac?
When the iMac was launched, there was pretty much nothing out there in the way of USB peripherals. Apparently USB was something Intel was keen on getting PC makers to implement for years, but it never happened 'til the iMac popularised/mandatorised* it.
* © Language-manglers'R'us...
As far as high-bandwidth peripheral connections go, I reckon FireWire has achieved 'critical mass' already.