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.
Software: Steinberg Cubase Artist 6.5.5 64-bit.
Hardware: HP elitebook 8530b with W7 Home 64-bit
Mackie Onyx 1620i - Steinberg Midex 8 - Steinberg CC121.
Analog: Alesis M1 - Behringer Powerplay Pro-XL - Studio Projects C3
Instruments: Yamaha DTX750k - Yamaha MO-6 - Waldorf Blofeld.
3rd party plugins: Native Instruments Prism - Rob Papen Distort - Rob Papen DelayMy humble contributions to what could be considered music.