that freighters don't have cabin doors to separate them from the hold.
Some configurations do, or did. When the SAA 747-200 Combi
Helderberg (on which a friend of mine's GF died) caught fire and ultimately came apart between Taipei and Mauritius the wreckage was recovered including the door between the pax section and the cabin. The handle had been smashed - presumably someone tried to get into the cargo section to extinguish the fire. But from the photos it wasn't a fireproof door by any means.
The report resulted in an impressive number of recommendations: I leave you to decide how many were actually implemented:
http://lessonslearned.faa.gov/SA295/SAAIB_report.pdf