I just held down the F key with the firewire enclosure attached and it booted from the enclosure.
As far as not being able to boot off of USB, if you can think of any other methods I'd be interested in trying them, but I honestly don't think it's possible. It doesn't show up as a valid boot option from the boot menu. If one goes into OF and tells the Mac to boot from USB it will start to load the installer for the OS but you end up with the gray "Stop" sign. It did this with 3 different drives in 2 different enclosures that are completely different brands so I don't think it was the enclosures or the drives. It def. wasn't the drive since I took the exact same one out of the USB enclosure, but it in the FW enclosure, and it booted up just fine.
As far as not being able to boot off of USB, if you can think of any other methods I'd be interested in trying them, but I honestly don't think it's possible. It doesn't show up as a valid boot option from the boot menu. If one goes into OF and tells the Mac to boot from USB it will start to load the installer for the OS but you end up with the gray "Stop" sign. It did this with 3 different drives in 2 different enclosures that are completely different brands so I don't think it was the enclosures or the drives. It def. wasn't the drive since I took the exact same one out of the USB enclosure, but it in the FW enclosure, and it booted up just fine.