For the differences in the specifications (yes, there are several types of FireWire connector), you may wish to do a quick search of your favorite search engine. You will find many articles at various levels of technical detail. One will, I am sure, be appropriate.
There was a time when all available iPods were FireWire based and could not even use USB. But as a relatively small percentage of PC users have one, keeping the iPod FW only would have hindered its availability to mac users and PC users savvy enough to install a card in their PC. Now, all current iPod models are USB only.