On the 340 (and I assume the 330 is the same, though I've never flown it so I don't know) if you shut the engine down with the master switch you shut both the fuel cocks (HP and LP), which would achieve what you're trying to do in the case of a fuel leak. If you use the Fire Button you shut off all the other services as well - hydraulics, air etc. - you also shut off the bleed air from the adjacent engine - very sensible if the engine IS on fire, but in other cases it might leave you with a few extra, unwanted problems. So I think on the 'Bus it's probably better not to use the Fire button unless you need to. Don't know how that ties up with the 744?
BTW - the Airbus QRH is not quite as clear as I made it above, so don't feel bad about yours! I was lucky enough to have it explained to me in words of one syllable by a kindly Training Captain - once he'd pointed it out, it became obvious what Airbus was trying to say, but at first reading it was quite obscure, at least to me
