I've heard of FireWire to USB adapters, but they are only for digital camcorders, not hard drives or other peripherals. Even if there existed an adapter, the only way you would be able to connect it to your computer would be through the USB port, thereby limiting it to USB 2.0 standards (speed, sustained transfer rate, etc.). With my experience, hard drive enclosures that support FireWire usually have USB ports anyway. You're better off using that than trying to find an adapter.