Quoting F111UPS767...
"Has anyone any information on an aircraft fire where the crew descended to 25,000 unpressurized and what the results were? How about manufacturer or government testing regarding fire suppression at 25,000 pressure altitude?
When descending to FL250 and following the cargo fire procedure, how long does it take to get the cabin pressure to 25,000? Someone posted that the 747 depressurizes at 2500 fpm. Is that correct? Is that the same for all the Boeings? Other manufacturers? Is it advisable or even possible to hurry that up manually? What would be the physiological effects of doing that? Someone posted a USAF study whereas 25,000 was chosen to lesson the chances of decompression sickness. What can we expect as to the possibilities of continued fire at 25,000 cabin altitude? What can we expect at 25,000 physiologically? Will a pack at 1/2 flow keep the cockpit warm? Is pressure breathing required? Et cetera...
I don't know what will happen with the lithium batteries at 25,000 feet. Besides the lack of oxygen, it should get pretty cold back there. If they do continue to burn, hopefully the stuff around them will not and the batteries may eventually burn out. Sound reasonable?
When at cruise at about 5-6,000 cabin altitude, if a major fire develops and an immediate descent is made, will the cabin altitude ever rise to a "fire extinguishing" level? Is it possible and should it be considered to put the fire out at 25,000 unpressurized before descend?"
Regardless of altitude the percentage of oxygen in the air remains at 21%.
So depressurizing an aircraft at altitude will have no effect on snuffing out a fire.
A fire will sustain itself with an oxygen content down to 16%.