Why does Montag now think about the sun and other things that burn?
By the end of the story, the readers can see the dual nature of fire/burning. Early in the story, fire and heat only represent one thing: destruction. The firemen burn away problems and use it as a tool of fear.
Throughout the story, things change. Montag uses fire to kill Beatty, and although it horrifies him, it came down to fighting with fire, so to speak, and Montag used the firemen's own tactics against them. This can also be seen in setting the alarm at the Black's home.
Towards the end of the story, we see Montag attracted to the fire in the woods like a moth to a flame. In this setting, the fire represents warmth and light; this is a much more positive association with fire than in the first half of the story. Granger and the others welcome Montag to their hearth and use the fire to keep warm and eat.
Finally, at the end of the novel, the city is disinegrated in a great fireball. This again is the violent destructive fire, but it is a great cleansing of the old failed society. It leaves hope for a better future.