You can boot from any FireWire drive, as long as the drive supports booting. Some older devices don't, but I think that most modern FireWire drives do support booting. I have three FireWire drives, and I've booted from all of them. One of them is a Lite-On 52x/24x/52x CD-RW drive which I boot from all the time to test boot CD's.
So the bottom line is, it's highly likely that you will be able to boot from your external drive. Only way to find out for sure, though, is to try it.