What do you need a Firewire PCMCIA for? the model of PB you have should already have Firewire on it - if you need more ports, Firewire hubs are much easier to run. (i.e. they don't need drivers)
If you really do need one, the Apple guy's advice is probably good advice - you need to make sure that it comes with Mac drivers, and if that doesn't, Apple will probably exchange for one that does because their salesperson recommended it. I bought a USB 2.0 PCMCIA card (my PB only had USB 1.1), and it did not have Mac drivers.... and I'm SOL.