Who does Montag think about as he looks back at the city? How does he feel about this person now?

Are you referring to the end of the novel? Montag looks back at the city and realizes that he gave it only ashes. Montag spent his life burning the soul of the city.

Montag remembers Mildred When the bombs begin to fall. At first he tells the other intellectuals by the fire that he feels nothing for Mildred but when the bombs fall, he thinks of the time they met:

I remember. Montag clung to the earth. I remember. Chicago. Chicago a long time ago. Millie and I. That's where we met! I remember now. Chicago. A long time ago.

This harkens back to a time when Montag did love Mildred. I think it is important because there was a time in their marriage when it was real. I don't see any mocking here.