The only thing Faber says at the time that he gives Montag his address is that Montag might someday decide to be angry at him. Later, at Faber's house, he says he thought Montag might come some day either with fire or friendship.
I think that there are two possiblities.
First, maybe Faber "sees" something in Montag that makes him think that Montag can be on his side sometime. He might think that having Montag as a contact will be useful.
A second option is that maybe he is sick of hiding like a coward. Maybe he is just taking a rash chance -- he's sort of thinking it might work, but he's sort of just wanting to do SOMETHING to prove he's capable of it.