If you look closely at the beginning of section three of the book, Bradbury has a very descriptive passage about Montag's feelings as he is forced to torch his own house. Montag feels a strange detachment--you might think that he would be super upset about having to burn down his entire house, and everything that he knew, but he almost seemed relieved. Bradbury writes:
"he wanted to change everything...that showed that he had lived here in this empty house with a strange woman who would forget him tomorrow, who had gone and quite forgotten him already...and as before, it was good to burn,...[to] put away the senseless problem."
This passage indicates that Montag actually felt good as he burned his house. It was getting rid of an empty life that meant nothing to him, of a house that no longer represented how he felt about living. It burned the entire past, including Mildred, who he realizes doesn't really love him, and gives him a fresh slate to start new with. He has changed so much from the first time we met him, and everything that his old life represented means nothing to him now. So, burning it is a sort of cathartic experience, unanchoring him from the "senseless problems" of his past. And, just like in the beginning of the book, burning was a pleasure, but this time, for different reasons. In the beginning, it was because he enjoyed the pure fantastic thrill of it; now, it is his symbolic phoenix going up in ashes, and he is free to rebuild. He is now free from it all, to act how he desires. I hope that those thoughts make sense; good luck!