Let’s say you put some slow burning rocket fuel in there, and it ignites one day. The inside of the safe will probably melt, smoke will ruin your entire house, but it shouldn’t burn down the whole thing.

Actually, this brings up an interesting alternate use for an old safe like that. If your old fireproof safe is broken, or you lost the key or whatever, you could turn it into a fireplace of sorts. Just flip it the door side up, throw in some logs and a place a large pan on top. May look a bit ghetto style, but who cares as long as the wok veggies and meat taste good.