They usually won't drop unless there's an open space in front of them or your enemy hits them down.
As for the back rows...^_^0
i typically dont move my heroes out of the fire, for the most part it doesnt hurt as much, besides...sometimes they expect you to, and when you do they screw you, so i just leave them in unless they are about to die from the fire damage
Now that you bring the topic upYou know who the god of fire really is nowadays? It's king Rod.