I hope you don't think that when the masks drop down they're automatically activated and tons of oxygen just start pouring out of them... you would be wrong. If that was the case it would be very expensive for airlines due to turbulence randomly making the bags drop down. You have to pull down on the mask before they're activated. Many of them have a re-breather bag that would limit the amount of oxygen that is released for these type of scenarios.
Dropping masks =/= opening a door.
I had a talk to a friend of mine who is a pilot and engineers the doors on airplanes. He said it's pilot preference. If there is a visible flame they generally will not drop masks, if it's just a lot of smoke they will.
It's not the masks they worry about, it's the fact that the system could be compromised and you just turned a small fire into a bomb. There is a line running to each of those masks which could be already on fire within the walls of the plane. Your pilot friend is right about it being the pilots choice, but they won't drop the masks unless it is a last ditch effort, in which case you are already dead and don't even know it. The key is to get to the ground. The masks can actually give passengers a safe feeling when really they are just dying faster. In a smokey cabin, the air on the floor is going to be better than what is coming out of that mask. The mask may contain more oxygen, but it can also contain more contaminates.
Doors don't equal masks, but physics tells me a fire will go nuts if you provide it a fuel source it does not have. It comes down to risking the deaths of a few passengers who suffocate or possibly killing everyone. Take your pick.