Are both computers connected to the Internet? I don't have a firewire cable, I just create a network that they're always both connected to so I can easily drag and drop stuff from one computer to the other.
On the computer you'd like to connect to, click System Preferences and go to Sharing. Make sure the first option, "Personal File Sharing" is enabled. Below that box of options, it'll say, "Other Macintosh users can access your comptuer at 123.456.7.8.900." Write down that IP Address and go back to your other computer.
In the Finder menu, click Go and select the last option in that drop down menu,
Connect to Server... Enter the IP address of the other computer and hit Connect. It'll ask you whether you want to connect to the other computer's harddrive or an individual user (usually doesn't matter unless your other computer has more than one user account). It'll put an icon on your deskop with your other computer's name and whenever you open Finder it'll also appear on the top left as if you had a CD in your harddrive. Your comptuer will remember the the last IP address you used to connect to a network, btw, so after the first time should you get off the network and want to get back on, it's as easy as clicking Connect to Server... and then hitting "OK" on the window that comes up.
Each computer can connect to the other and you can watch movies, view PDF and photos, etc. without even needing to actually drag them onto your computer (and if you do want to bring them over, it's easy as drag and drop). It's a much better way than using firewire cables because a) you don't have to be physically tethered to the other computer and b) you don't have to re-start your computers any time you want to exchange files and programs between the two computers.