USB requires a host. Firewire does not. So, to go USB to Firewire, you need a host, the PC.
What are you trying to do? Do you have a camera with Firewire? There are cards for under $40 available. Then you need a cable. You just cannot get Firewire into a USB port directly with a cable. Your only other option is S-video or RCA, but you need something to do video in. Here, you would spend as much, and lose quality.