On the Boeing Freighters you do not descend to FL 250.
Item 7 on our B744F QRH checklist for FIRE MAIN Deck states:
-Climb or descend to 25,000 feet when conditions and terrain allow.
This step is done immediately after pressing the CARGO FIRE DEPRES/DISCH switch which does indeed among other things depressurize the aircraft to a cabin altitude of 25,000' or airplane altitude, whichever is lower, but the checklist still has you climb/descend to 25,000'.
My best guess would be that, as the cabin is still pressurized to 25,000', having the aircraft at the same altitude would decrease Delta PSI to almost zero and thus diminish the flow of air within the pressurized hull, further discouraging combustion.
MD