USB and FW are inherently different. Everything I have read indicates a FW to USB adapter will not work.
I have seen a few places that say it will, but they admit they've never used one. I have never seen a single forum post, blog post, review or anything else by someone using a USB to FW adapter that says it does indeed work.
I have also never seen such an adapter on a single, what I would call, reliable or at least semi-reliable site. No such adapter exists on newegg, tigerdirect, bestbuy, fry's, amazon, monoprice, owc, or any other site I can think of real quick.
But hey, for $6.95, why don't you get it and report back to let us know if it works or not. I am curious and sure there are others beside myself that would like to know from someone that has actually bought one of these things.