On the Boeing Freighters you do not descend to FL 250. When you arm the fire suppression system for the main deck the pressurization system dumps the Cabin and maintains an internal pressure equal to FL 250. This allows the aircraft to remain at altitude for best range and quite possibly best forward speed.
Speaking for myself, if I have fire and the regulated cabin altitude of FL 250 is not suppressing the flames, maintaining your original cruise altitude will permit raising the cabin to a higher altitude an possibly suppressing the fire faster or retaining a greater margin of time required for the emergency descent to landing.
The ideal situatuation is to stay as high as you can and as rapidly as possible dive to your intended diversion airport or controlled point of impact with the water.
Even with a battery fire, the batteries have very limited amount of energy to release. Once they have shorted out the lack of oxygen in the cabin (in theory) should provide you with enough time make an emergency descent to landing.
If the inferno damages the airframe, the low pressure on the pressure vessel should help to retain some or most of the structural integrity.