If you've ever stood near an open fire, especially when it's cold outside, you've felt the "fiery smile" to which you refer. It's that feeling like the expression on your face is going to be fixed in place by the heat from the fire. The narration refers to this smile on Montag's face in the book's third paragraph. Montag feels this burnt-in smile because he's been a fireman for a long time and as the opening sentence states, "it was a pleasure to burn", meaning that Montag liked his job. It was what he was trained to do and all that he knew to do. He'd been indoctrinated, as had most of the people in his society, to believe that what he was doing was the only right thing to do. He'd been brainwashed to believe that their society was the correct society. It wasn't until after he met Clarisse that he really started to think about things. His curiosity was always present (we know that because he's already had books hidden before he met Clarisse since when he got home after having met her for the first time, he looks up at his ventilator grille where the books are hidden), but he never did anything thinking about it or about why he was curious. Once Clarisse planted that seed in him though, with the question, "Are you happy?", Montag does begin to question his life and his society. When he starts this questioning and he starts to look at life through clearer eyes, he sees their society for what it is. He understands that they are all blind and isolated because they don't think, they don't read, they don't interact with one another on a personal level. By the time Montag goes to Faber to enlist his help in getting copies of books made and in bringing down their society, Montag is pleased that the burnt-in smile is gone. He knows that now, if he smiles, it is not fixed in place by the heat of the flames. He knows that any smile he makes now is made from freedom and not fire.