At the beginning of Part Three, Beatty tells Montag to burn down his house with a flamethrower. Initially, Faber (listening in on the conversation) asks Montag if he can run away, but Montag knows he cannot escape because the Hound is somewhere in the vicinity.
Despite some initial reluctance, Montag has no choice but to go inside with the flamethrower, and his reaction to this task is surprisingly positive, as we see in the text:
And as before, it was good to burn, he felt himself gush out in the fire, snatch, rend, rip in half with flame, and put away the senseless problem.
Setting fire to his possessions is, perhaps, cathartic for Montag. As he watches his home burn, he is released from the burden of living a double life in which he had to balance his duties as a fireman with his increasing desire to bring censorship in his society to an end. Thanks to the fire, he no longer has to worry about hiding his books, nor does he have to listen to the constant noise of the parlor walls. His marriage, too, is over, since Mildred has already left.
Burning his home gives Montag a second chance. He can now put his plans with Faber into action. But, in order to take that chance, he must first evade Beatty and the Hound.