The AirPort slot is not a PC Card slot. It almost is, but it runs only a subset of the PC Card standard.
I have experimented with non-AirPort cards in an AirPort slot. The only one that worked was a Lucent WaveLAN silver card, since that card is the one the original AirPort card is based on. All the other cards I tried weren't even seen by the computer.
Besides, you couldn't fit the FireWire card inside there. There's not enough room for the part of the card that normally protrudes from the slot!
On top of it, I don't know whether a Mac can boot from any FireWire that's not on the motherboard. I doubt it, considering that not even all Macs with FireWire can boot from their on-board FireWire!
tooki