Per the OP's request:
It wasn't that long ago that the AMSEC RF6528, and its lack of a fire seal was being discussed. I had some photos of similar composite safes that had been involved in a fire, but none of an actual AMSEC. This is a safe that I sold to a customer new. They called me the other day while the house was still on fire, so in addition to the photos, I know the specific details.
This safe was located in a walk out basement, in a utility room area where lawnmowers and ATVs were parked. The fire was caused by gasoline fumes ignited by the water heater. The owner was home at the time, and luckily, him and his family escaped without injury. The house and everything in it was a total loss. You can see the top of the safe in the middle of the photo:
The safe has a factory 2 hour rating. The fire department had a hose on the area that the safe was in to attempt to keep it as cool as possible. The fire burned for 8 hours, and the safe sat for another 2 hours until they could get a chain around it and drug it out of the smoldering debris with a tractor.. This is what was left of it:
As you can see, there was very minor damage to the contents. Not bad for a safe designed for 2 hours being exposed to 8 hours worth of heat. There was a bit of soot inside the safe, along with some minor water intrusion, likely from the fire hose. All of the important items inside the safe survived. The envelopes protected their contents from the soot, and all of the paperwork inside was in great shape. Paper money survived fine. Some jewelry and coins survived fine. Some of the plastic containers didn't do so well, and started to deform. Nothing melted. Note to those using plastic containers: Make sure they are microwave safe. The plastics on those containers require a much higher temperature before they begin to melt.
Overall, in this case I do not think a door seal would have made much of a difference. The safe was exposed to heat for far longer than it was rated, and still peformed fairly well. Had the safe only been exposed for 2 hours, I suspect what little damage there was would have been less.