All of the conversations that Clarisse has with Montag can be found near the beginning of the book, so that will help you to locate where they discuss this topic. The first time that they meet, Clarisse immediately guesses that Montag is a fireman--she can tell because he smells like kerosene. They begin to talk, and eventually, as they get to know each other better, she remarks how he is not like other firmen, and how she thinks that he doesn't really fit the stereotype. The reasons that she gives don't really paint firemen in the most flattering light. The first reason she doesn't think he fits the stereotype is that, as she puts it, "I'm not afraid of you at all." To her, firemen are terrfying people that show up only when your entire life is about to be destroyed. As a result, people are pretty afraid of them, especially a family like hers, who doesn't really fit the mold. So, Montag isn't scary, for one.
The second reason that she gives for Montag not fitting the mold of a fireman is that
"when I talk, you look at me...the others would never do that. The others would walk off and leave me talking. Or threaten me."
To sum up, Montag actually listens to her when she talks, and thinks about what she is saying, whereas other firemen are impatient with her ramblings, don't care a hoot for what she says, don't listen to her or think about her questions, and often threaten her when she is talking. Montag, on the other hand, listens, asks questions in return, cares about her thoughts, is kind, and seems to think about her comments as much as she does. This doesn't really fit the mold of firemen, or most other people in her society at all.
Clarisse's comments about him not really fitting as a firemen turn out to be almost prophetic; after meeting her, he finds that his joy in his work diminishes, and in the end, he will reject the lifestyle all together. I hope that those thoughts helped; good luck!