Pressure
Does pressure make any big difference in firefighting?
I think that the amount of oxygen per unit volume drops with pressure decrease - and so does heat capacity per volume, cannot see the direction of that effect. The viscosity and heat conductivity of gas should be independent of pressure.
Of course, opening cockpit windows to blow smoke out of cockpit can only be done if the plane is unpressurized...
PSU is set to operate at 14 000 feet, so that would be the maximum height possible - automatic deployment would make a major addition to fire intensity.
Can airliners ditch in one piece?