Have: iPad 3 (Apple 30-pin connector) PreSonus Firestudio (firewire 400 output)
How to connect them?
You don't. Firewire isn't compatible with a full fledged USB controller and the 30 pin connector isn't even a full USB connection to connect peripherals.
Sell the Firewire device and buy a device that will work with the iPad is sadly the best advice I have for you.