That quote appears in the opening lines of the book. At that point in the novel, Montag likes to burn stuff. "It was a pleasure to burn," Bradbury writes. If your question is asking about which of the five senses (sight, sound, taste, touch, smell) that the quote is appealing to, then it most likely appeals to the sense of touch. Montag isn't touching the flame, but he can feel its heat. If you have ever stood close to a campfire, you know what Bradbury is describing.
What happens when a person stands close enough to a fire to get singed is the pain response. When a person is in pain, the typical facial response is a grimace. The neck muscles tighten up, the corners of the lips pull back and up, the eyebrows furrow, and the eyes tend to squint. From a distance, this facial expression would kind of resemble a smile, because of the lip and cheek movements. Montag isn't grinning/smiling at this point in the novel. His face is reacting to being close enough to the fire for his body to feel pain. That's why he is grinning/grimacing. He can feel the fire.