Cisco's GigaStack cable uses connectors that visually look physically identical to a 6-pin FireWire connector with the addition of a special gasket-like ring on the outside that appears to help it mate with the socket and probably provides better EMI shielding.
Cisco states that you should not interchange a GigaStack and FireWire cable.One of the reasons is the pinout of the cable itself. Pins 1 and 2 are shorted to each other electrically. This probably provide a way for the GigaStack GBIC to identify a real GigaStack cable versus a FireWire cable.
Otherwise, the pinout of the cable appears identical to a 6-pin to 6-pin FireWire cable by testing an actual cable.