It's the smoke that affects crew and pax first. The advice to switch off the 2 packs and allow the aircraft to depressurize to 25,000ft, is to minimise the oxygen flow and suppress the fire, however, you still have, statistically, only about 15-18 minutes to land the aircraft (either on land or by ditching), if the source of the smoke/fire cannot be extinguished. As for dangerous goods on freighters, perhaps you should ask security agents what items they discover in pax hand luggage! These are, of course, undeclared and ill-packed dangerous goods.