Because of his job, Montag is very wedded to fire as a destructive tool. He, like others, has been indoctrinated to believe that destruction is a good thing, and that his actions are keeping society from panic and chaos. However, the symbolism of fire changes for Montag; he stops seeing it as "good" and starts seeing it as "bad" as his own individuality develops.
Essentially, Montag has been conditioned to see fire as an equalizing force; everyone in society should be the same, and he keeps this under control with his fire. However, as he becomes an individual thinker, he starts to see how fire destroys and does not create; creation is a high calling for Mankind, and fire, as an uncontrolled tool, simply erases all of that creation. At the end of the book, with his mind fully freed from the dystopian society, Montag has two epiphanies: first, that the burning of all things by time is inevitable, and that he should spend his time trying to create; second, that fire, as a tool, is not just for burning, but for defense, warmth, cooking... it becomes just a tool of Man, not a higher force.
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!
(Bradbury, Fahrenheit 451, hrsbstaff.ednet.ns.ca)