I'm no computer tech, but I don't think it will work for you. USB and firewire each have their own protocols, each require dedicated hardware, drivers, controllers, etc...
I use firewire a lot with my setup and can appreiate why it's better than USB for audio (hot swapping drives, better transfer rates). You might want to ask a tech if there's a possibility of installing a firewire card (maybe the USB is a discrete card rather than built into the mobo). Good luck.

Thanks for the input Jon,
It doesnt seem like a good idea to me, but my brother says he has such a conversion jack that has worked sucessfully in other situations. I dont want to spring for the firewire unit and then discover it wont work this way, as you can imagine. The laptop is also a few years old and I dont fancy investing in a modification.
Derm

your laptop likely has usb1.1 (being that its older). your hardware that is using firewire is going to require a lot more bandwidth than usb1.1 could offer anyhow... that is why there is usb2.0... regardless of the valid reasons posted b4, you couldn't use a firewire jack into a usb due to the speed difference... which is something like 40x

Firewire PCMCIA cards a super-cheap. If you've got an open slot, you're good to go.

Derm:

Your best bet would to just buy a firewire cardbus and use it for firewire slots. I had this problem with a USB external hardrive and it wouldn't playback the audio fast enough so I ended up buying a firewire drive like a month later. I haven't had a problem with audio playback since. Just buy a cardbus and you should be okay with slots. Check out CompUSA, Circuit City or BestBuy.