I would be most surprised if this worked for anything but supplying power to a device.
Firewire and USB are totally different technologies and it would require some smart logic built into the converter to make the conversion transparent at the protocol level. I can't see a device that small would have room for such circuitry.
Then there's the issue of drivers...
If you can build a bluetooth adapter in a USB connector I don't see why you can't built a couple of tiny chips into an adapter plug.
Why would they advertise it if it wasn't a convertor ?
if it was something like this:
http://www.usbfirewire.com/Parts/rr-527950.html
id say it may be plausable.
usbfirewire.com said:SOLD OUT
The manufacturer has decided not to continue production
No other similar items available