4.3.1 What exactly is a fireball?
According to Steve, an orange-sized bolt of magical force that hits like a bullet and which quickly vanishes in a blast of fire on impact, detonating any flammables it actually strikes.

What temperature is a Fireball?
About 300 degrees centigrade (570 degrees Fahrenheit).

What kinds of wounds does a Fireball cause?
It is largely an impact weapon; a baseball-sized bolt of pure magical force. This is why it's damage is stopped by the DR of the location hit. It is hot, and it does vanish in a gout of flame when it hits the target, but that's more of a special effect than its primary mode of doing damage.

How does a Fireball interact with armor?
This «hard» bolt of magical force hits the armor just like a bullet, staff thrust or fist, doing damage on impact, and either denting it a little or punching a hole straight through. It then goes «Poof!» and vanishes in a cloud of fire. It will only heat up a breastplate as much as a baseball-sized globe of 300-degree air would; i.e. not much.

Does unsealed armor protect against an Explosive Fireball?
Yes. Here's how it works:
The Explosive Fireball does flame damage – treat as crushing damage that can set fires. It is not a true explosion, but damage decreases with distance in a manner similar to explosive concussion damage. The target may attempt to dive for cover to get away from the center of the blast. DR protects normally; assume torso DR unless the target is hit directly by an Explosive Fireball aimed at another body part.