What is the significance of the scene in which Montag first meets Clarisse?
In my opinion, the significance of this scene is that it represents the start of Montag's transformation in this book. After meeting Clarisse, Montag starts to think and starts to reconsider his opinions about the society in which he lives.
When he meets Clarisse, Montag still thinks of kerosene as perfume. This symbolizes (to me) that he totally buys in to the ideas of his society. He thinks that the smell of the stuff that is used to burn books is great.
But as he talks to Clarisse, he starts to have to think. She is just too different from other people in his society. By the time they are done talking, he has thought about a tender moment with his mother and he has started to have to consider whether he is happy.
In other words, he has gone from being a contented member of society to being confused -- he has had to start to think about things like his emotions, things that have no place in his society.