Montag's entire association with fire has been as a tool of destruction. He knows how to efficiently soak a building with kerosene and burn it to the ground, even though houses are fireproofed, and when the book begins he takes joy and pride in his work burning books. As he becomes more interested in the books and develops an individual identity, he starts to think of fire as an evil tool of destruction. When he finally escapes the city, he comes across a "hobo" camp, composed of people who memorize books:
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!
(Bradbury, Fahrenheit 451, Google Books)
The men in this camp use fire to keep warm and to cook food. They have tamed the fire, and are using it in the pursuit of their constructive goals. Montag finds the fire almost alien because it was not set to consume, but to improve; the men around it are happier because of the fire, not in spite of it. For Montag, the fire represents the final severing of his life as a fireman; he will no longer think of fire as a force of destruction but as a tool in servitude to mankind.