Man. I thought it was an ironic thread. But obviously not. I do not even understand how you can get two-three month old firewood to even burn.
I have several extinguishers close at hand, one is a carbon dioxide unit. I would think it would work well on a chimney fire as it cools as well as displaces oxygen.
While taking some firefighting training our instructor, a retired firefighter, told us rip the top off a box of baking soda and stick it in the flu. The up draft will suck the powder up and smother the fire. Since baking soda is the primary ingredient in dry chem extinguishers I took it a sound advice. Not that I've ever tried it
Burn only wood, seasoned wood, don't choke your air supply down too soon, and you'll be in a better place. Clean your chimney every year if you use the stove on a regular basis.