What was different about the fire Montag saw after leaving the river ?

The fire Montag saw after leaving the river was different because it was a warming fire rather than a burning fire.

But the fire was there and he approached warily, from a long way off. It took the better part of fifteen minutes before he drew very close indeed to it, and then he stood looking at it from cover. That small motion, the white and red colour, a strange fire because it meant a different thing to him. It was not burning; it was warming!