Around the beginning of the book, Clarrise asks Montag if he ever reads the books that he burns and Montag answers by saying that that's against the law and he doesn't. This foreshadows later in the book when Montag starts to wonder what is inside of books and starts to sneak them in his house a reads them.
Do you ever read any of the books you burn?
No! That's against the law!
When Montag first meets Clarisse they have a bit of a deep conversation. Before she left, she left Monatg with this question: Are you happy? - Montag tells himself that of course he is happy and that that question was a dumb question. But once he gets home, he starts to think about it and slowly realizes that he isn't happy. This foreshadows the drastic change Montag would go through in the book and would start to question everything.
Are you happy?
Haha what? Of course!
Or maybe not?
The Mechanical Hound being aggressive to Montag and Montag not feeling safe around the hound at the beginning of the book foreshadows the ending when Montag is in trouble with the law and is being chased by the hound.