I'm getting rather tired with all this Firewire moaning and moaning and moaning. Buy and adaptor for God's sake!
I really doubt an adapter is going to work well, if at all, in most cases. These are two completely different serial protocols. I don't see how a device can speak Firewire while the computer speaks USB with just an electrical conversion to do the translation. These protocols are implemented by considerable software that expects the other end to speak the same language.
For sure you won't be able to get target disk mode with an adapter, which is what I'd be looking for.