In the UPS 747 crash, it was Lithium Batteries. The Malaysian 777 was also carrying Lithium Batteries in the cargo hold. I don't know how many, but it only takes a few to start a fire. HOWEVER... the 777 has a pretty good smoke/fire suppression system for the cargo hold, and that should not affect their radios.

The UPS 747 was talking on the radio right up until they hit the dirt. That's why I think the Malaysian problem started with their radios, or something in the E/E bay, which has no fire suppression.

Once you have depressurized the airplane (at any altitude) you can indeed open the **** windows, but not the cabin doors, as they swing forward, against the wind, like your car doors, but you'd never be able to force it open with a 200+ knot windflow.