Lots of good info in Libertarian01's post. Just a couple of thoughts, though...
Plastic - What about polymer pistols, or guns with plastic grips or stocks? In other words, you can't completely avoid putting plastic in the safe.
Ammo - If you don't put ammo in a safe, where can you put it? If you leave it unsecured, it's accessible to any kids in the house, and easy pickings for a thief. If you lock it in a cheaper metal cabinet, it will take a thief a little longer. In either case, the heat will get to it faster than if it is in the safe, so any risk associated with the ammo getting hot will occur faster during a fire. There's only so much that you can do with ammo.
Has anyone installed a sprinkler system in their home?
Great points Libertarian, especially about paying a little more to "be" safe, not just "feel" safe. It's amazing that some people want to stick tens of thousand dollars worth of guns, cash, jewelry, etc., in a single consolidated spot but don't want to spend more than a few hundred to protect it.