I'm trying to give you whatever clues I can to help you find the answer but don't know it myself. I'm not intimately familiar with how the connection between macs works. But I do know a tiny bit about firewire, including that it acts like a network connection when you plug it between computers.
A mounted file system is a file system you see in the shell like /path/to/files. To access the remote files you'll need to mount them somehow.
[edit] goggling "apple file sharing" showed me that OSX may be using AFP (apple filing protocol) for this. There are ways to mount AFP from the shell. See
here