Try as I might I can't really understand this. Maybe you can describe where each of these machines is located and how you connect to the internet, and why it was necessary to share internet over firewire in the first place. It just sounds bizarre to me right now without seeing the whole picture.
If they're in the same room which I would think they'd have to be if you were connecting them with firewire then I can't see why a router wouldn't work. Wireless reception would be fine OR you could just do it wired.