In the book Fahrenheit 451, what was different about the fire Montag saw after leaving the river?
Prior to this moment, Montag has only known fire to be destructive. He has spent much of his life as a fireman starting fires in order to burn books, and while at the beginning of the novel he took pleasure in this occupation, he has since learned the value of books and wants nothing to do with his former occuation. While floating in the river to escape the mechanical hound after having killed Captain Beatty, Montag decides that he never wants to burn again - he wants to make a complete break from his former life.
He is surprised, therefore, when he emerges from the river to see a different kind of fire - one that provides warmth. He sees the men in the forest - many of whom are former professors and intellectuals - gathered around the fire in order to seek its light and its heat, and as they sit together, they enjoy each other's company and conversation. Montag "hadn't known fire could look this way. He had never thought in his life that it could give as well as take. Even its smell was different," for the fires he had known before were started by kerosene, and this one was caused by burning wood.
Montag now realizes that fire iteself is not the enemy - it's how it is used. He now joins this group of "book people" and agrees to be one of them. He will be the "Book of Ecclesiastes" and will, together with the others, try to reintroduce the knowledge and wisdom that come from books into society when the time is right.
That time comes soon enough when the bombs drop on the city, leaving a giant path of destruction. Granger now teaches Montag about the phoenix - the bird that bursts into flame then rises from its own ashes, renewed and replenished - and Montag learns that fire can be a cleansing agent as well. Now, perhaps, with the city having been cleansed, the surviving inhabitants will be ready to make a new start.