What old feeling came back to Montag as he burned his own house?
I think the "old feeling" you are asking about refers to the way he used to feel about burning houses down. Before he met Clarisse, and before the old lady killed herself, Montag used to like burning down houses. Now, as he burns down his own, he gets that feeling back again. Once again, it feels good to be burning things down.
The difference, of course, is that now it feels good because he is burning down a part of his life that he now hates. He is burning the loveless relationship that existed between him and Millie.