You might try a search for FileGuard or DiskLock, which might lead you to something, but I don't know if those work in OS X. Also, do a Google search for "mac security" and you'll see a number of sites that specialize in that topic, like securemac.com
I've never tried this, so I don't know if it would work, but you might try using Disk Copy in OS X to create an encrypted volume on the Firewire drive. Go to Disk Copy > Image > New Blank Image and create a volume on the Firewire drive, as big as you want (use Custom size). Use encryption. After you've created the volume, just double-click on it to open it, and it will ask you for the password. It will then appear as a separate disk/volume on your desktop. When you want to close it, just drag it into the trash.
You might have to format the drive in OS X, or install OS X on the drive - I'm not sure.
If I'm not mistaken, this will allow anyone to access the Firewire drive, but they won't be able to access anything on the encrypted volume without the password. And as far as I know, there is no way to bypass this. But again, I've never tried this on an external drive, so I'm not sure if it will work.