Should depressurization be considered in the event of an inflight cabin fire?
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.