My motherboard has one slot for Firewire actually on it to plug the 4 ports I got in the box into a spare back slot of the computer.
If you don't have this on it, you'd have no choice but to just buy a PCI card to add this feature on which the motherboard didn't include.
I definetly doubt one Firewire to USB adapter would make it have firewire functionality as it would be easier to just get one of those than for people to buy seperate cards...
What ColdWaterCS is getting at I think is that there is a connection for it on the case, or a space for one, and yet doesn't have the functionality yet to pull a cable through to this connection to allow it to be used, as some cases do come with them so taht if the motherboard has a connection for it, they can just get a Firewire wire and plug this up from the motherboard connection to the spare slot on the case