I don’t think Firewire to USB exists, and if it does I’m sure it’ll introduce some extra latency at least, or incompatibility problems at worst.
In other words, don’t buy a Firewire interface if you don’t have a firewire port :p.
You may have a small 4-pin firewire input on your laptop though, for which there are adapters which work fine.
If your laptop has a PCMIA slot, you can get a firewire card for that. Make sure it has a Texas Instruments firewire chip.