Fire is one of the most powerful symbols in the book. It is seen as a destructive force almost greater than Man; it is used to quell opposition and solve problems by destroying them. For Montag, fire is his entire life; his motto is "It's good to burn." Slowly, though, he stops loving fire as his life and starts to recognize how it is a negative in life, not a positive.
...if he burnt things with the firemen, and the sun burnt Time, that meant that everything burned!
One of them had to stop burning. The sun wouldn't, certainly. So it looked as if it had to be Montag... Somewhere the saving and putting away had to begin again and someone had to do the saving and keeping...
(Bradbury, Fahrenheit 451, hrsbstaff.ednet.ns.ca)
Fire there is a symbol for the passage of time; the sun continues regardless of other events, simply because it is so large, but time is valuable to the short-lived people on Earth. Time is burned up by waste and sloth, and Montag has not only burned his own time, but the time of others. Now, he can put his own time to better use; he can create, which allows others down the road to better use their own time; Montag's fire will not be destructive any longer, but instead change from burning time to "cooking" it, making more time for himself and for others through his actions.