Samizdata - 9:02 pm on Apr 5, 2011 (gmt 0)
I wouldn't recommend using the external as the boot drive
While this would certainly not be a general recommendation, I frequently use external FireWire drives to boot Macs (including the one I am typing this on) and have never found any drawbacks other than the reduced portabilty of laptops.
And I would definitely recommend installing OSX on an external FireWire drive if you have one. Put it on a partition of its own, it may come in useful someday.