Yes, it can be done, I get Internet and File share via Airport, Firewire, and USB Wifi adapter at the same time.
There is something a bit messed up with yours, and it may interfere with this method.
Start by Repairing Permissions and rebooting. Then Make a New Location for this setup.
Write down the IPs and Router IPs of both when each are at the top & working for the one thing.
In Network>TCP/IP tab>Configure IPv4, set it to Manually for each Interface one at a time, filling in the Internet working numbers for Ethernet and Apply, then goto the Firewire Interface, switch to Manually, fill in the numbers that worked for File Sharing, Apply.
Does it work this way?