Many many years ago I used to work at a GM proving ground. We were taught there that if you have an engine fire make sure you pull the hood release (from inside the passenger compartment) as soon as possible, but do NOT try to open the hood (or release the under-hood lever) because the rush of oxygen will make the issue even worse. They told us to call pull the interior release, step away from the car, call the fire department, and wait.

I guess the fire can damage the hood release cable/connector and make the hood almost unopenable if you don't pull the interior release rather soon. If that happens all they can do is try to fight the fire from underneath - which has only limited effect.