Should depressurization be considered in the event of an inflight cabin fire? Uncontrolled cabin fires only need a few minutes to spread throughout the cabin of an aircraft, killing everyone onboard and destroying the airframe. 

Now, we all know that depressurization and fire do not combine; the lack of oxygen will eliminate one of the elements necessary to keep the fire alive. So if an aircraft had a way to depressurize the cabin and eliminate oxygen in the event of a fire, the fire would be extinguished, saving lives. Of course this applies to flights above 14,000 ft. I know that depressurization is dangerous but there is a better chance of surviving a depressurization than surviving flames.

Does a mechanism exist to depressurize the aircraft in case of uncontrolled fire in the cabin? If so, under what circumstances would it be used?
Some aircraft do have a manual decompression button which can be used by the pilot in a situation like this.  Keep in mind however that it will only help until the pilot descends (which you will want to do asap in order to land) and the air isn't as thin.  

It is very important for the crew to get on the ground as rapidly as possible, and simultaneously attempt to put out the fire in order to minimize the amount of damage that can be done.  The single biggest factor in an aircraft fire is how much time elapses before they land and can evacuate.

Also, if the cabin depressurizes, typically the oxygen masks deploy automatically which when combined with a fire can have even more disastourous results.