Assuming your computer can boot from firewire, you can just install FreeBSD to your firewire disk as you would install a normal FreeBSD installation.
You can do so with the FreeBSD installation disk, or if you have a running FreeBSD installation, you can use sysinstall and choose your firewire disk as destination drive.
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UNIX was not designed to stop you from doing stupid things, because that would also stop you from doing clever things.
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