Fire Extinguisher In Unheated Garage & Shed
Anecdotal I guess, but I've had a fire extinguisher out in my barn for years now and it's never been a problem.  Being in Michigan, it regularly gets below 0F.  Granted, I've never actually tried to use it, but I think the freezing point of the chemicals in there is way below zero.

ETA: 

Looking at this page it seems like the operating temp of a typical ABC extinguisher is going to be -65F to 120F.