I believe the point alpha was making, is by getting a "fire-rated safe", you'll hopefully have temperatures inside the safe low enough that it doesn't hit the ignition point of the ammo inside. For ammo, I follow the recommendation of a late friend of the family. He was the fire chief for 25 years and a fire investigator for another 10-15 years. He said from their perspective they'd rather the ammo be inside a good, high temp rated fire safe. In absence of that, then they'd rather loose ammo in boxes. What they don't want is ammo loaded in the chamber in guns, and especially not in a tight fitting box with no temp/fire rating.
Here's how he explained it to me.
With a proper fire rated safe, they likely won't know you have ammo or a safe until the end. While it is true that smoke will damage paper, photos, and firearms before it gets hot enough for the seal to expand and seal it up, that seal should expand before inside temps get high enough for the ammo to hit its ignition point.
When they are fighting a residential fire, the little pop pops of small arms going of individually gets their attention, but isn't particularly concerning. Since the pressure is not contained, it is not dangerous. It just goes bang and sounds like the cheap firecrackers.
However, these last two are dangerous. A loaded gun that has a round "cook off" is dangerous because it is under pressure, and projectile(s) are directed the way it is pointing when it goes off. This is no different than a normally fired firearm.
The most dangerous is when you have a stockpile of ammo in a fairly tight sealing lockbox. The reason by now is probably self evident. Once you hit the critical ignition temperature, it will go off. This can, will, and has caused them to all go off in very quick succession. Do to contained pressures, it effectively results in a bomb like explosion.
They are trained to be able to tell the differences of which situation it is just from the sound. He said they would always withdraw on the last two unless it is believed people are inside needing rescue.
I keep my expensive ammo on the bottom of my safe (where temperatures will be lowest). The rest are in those plastic ammo containers for rifle ammo, original boxes for most of my pistol ammo. Only my bulk 10mm auto ammo is stored loose together. Most of it is in coffee cans and such. I do have a small amount of ammo stored in a couple army ammo boxes which are the least safe option I use. I don't keep the cans together. This is my "grab and go" or bug out should SHTF scenario ever happen. All told, including a lot of .22lr ammo, I probably have 100k rounds of loaded ammo. I am nearly out of projectiles, but I have enough primers, brass, and powder to load another 25k. Prior to 2020, I used to shoot at least 10k per year so I always have tried to keep on hand 3-4x the amount I expect to shoot each year. I stocked up more than usual early on as I thought there was a good chance ammo would go up in price globally during the pandemic. As the 2020 elections neared in the US, I correctly suspected it would go up even more and might stay there for quite some time depending on which POTUS was elected.
While it hasn't opened yet, we are a month or two from having a fire department station literally 1/4 mile from the house. The irony to it is that we live in the country about 7 miles from town. It is an odd location, but there are a lot of oil and natural gas drilling in the area so that is probably why.