The fire will spread if the chemicals are oxidizers.
And if there are no oxidizers in the mix? Or a small amount of oxidizers? Or for that matter the fire has started in a different part of the load?
Things do not like to burn at altitude, and we should not ignore the one trick that we have up our sleeve. So I am following the checklist while getting pointed to the nearest airport. And then I am going to sit at altitude, unpressurized, until the last possible moment. Then, and only then, will I start down like a brick for the runway. Gear down, speed brakes out, Vle (+ a bit). Like a Baron full of Doctors.
The ability to depressurize and remain at altitude is a huge advantage that freighters have over passenger flights. It is not a great solution, but it is worth more than doing nothing. And it is certainly better than going down early.