You are welcome...I guess the comment you made begs the question of what were you really after with your initial question...if it is only can you boot ML from Firwire 800 the answer is YES. If it was can you boot and how do you create a bootable external drive, the second part is a little more complicated. If you have Snow Leopard or Lion on the internal drive, just download a copy of Carbon Copy cloner or SuperDuper (CCC is no longer free but you can use if for free for 30-days), and clone the internal drive to the external. Then boot from the external and go to the Mac App Store and download (for $20) the ML installer. After you download, don't install right away...make a copy of the installer (should be in your Applications folder) in another location such as your Documents folder. The isntaller self destructs as the last step in the installation.
Then run the installer so that you end up with ML on your external drive, which is already bootable and remains so.