I've never had to use a chimney fire extinguisher, so take this advice for what it's worth...
When we had a woodburning insert (QuadraFire 2700i) installed back in February, I looked at chimney fire extinguishers. The Chimfex extinguisher is almost like a flare. You're supposed to go through a 3-step process to ignite the thing before chucking it into the fireplace (next to the fire, NOT directly into the flames). See:
http://www.chimfex.us/resource-center/how-to-use-chimfexr.html
Instead, I went with a simpler extinguisher. I bought Meeco Red Devil FireEx. It is basically just a sealed plastic pouch containing the extinguishing agent. Open the fireplace door, chuck in the extinguisher (pouch & all), close the door. Caveman simple, easy for my wife or children to do in a stressful situation. We keep it hanging on the mantle...just in case.