the firebox comes with a cable to plugs into that 9 pin serial port that has 4 dongles (standard 5 pin MIDI in and out, and RCA S/PDIF in and out).
they made it a serial port for the sake of compactness, since having 2 MIDI jacks on the unit would have to increase its size by almost 50%.
i'm fairly certain the OP isn't concerned with MIDI, S/PDIF, or ADAT compatibility since it sounds like he's only looking for a few line and mic channels direct to PC.
i disagree about the pre-amps, though. the ones in the firebox *ARE* very good. not the best, but easily better than anything else in the price range.
(on a side note, i didn't know the firestudio had different pre-amps until now--the firebox has the same pres as were in the firepod/fp10, but i guess they upgraded them when recently when switching to the firestudio line)