FWIW, the procedure for dealing with fires in cargo aircraft (in areas that do not have extinguishing)is to depressurize the aircraft and fly at FL250. They pick FL250 as a balance due to oxygen concerns, time to get on the ground, etc. According to people that are knowledgeable on the subject, it works. I asked ALPA's dangerous goods specialist this question recently, actually, and he confirmed that the procedure was sound as long as the material causing the fire did not generate its own oxygen (obviously).