Having a "lookee-see" on a 2-man crew freighter may impose a very high workload on the remaining pilot, and what exactly would you achieve? On a fully-loaded freighter, the space to determine where the source of the smoke/fire, is very limited and vision will not be too good with the smoke mask/goggles on. Better to seek a near landing site and proceed at high speed, while communicating your plight with all and sundry. After all, if Boeing consider 25,000ft to be the cabin altitude at which the fire will be suppressed, how long would you stay at that cabin altitude before commencing a rapid descent into thicker oxygen, which may cause the fire to flare up again?