This quote is found in the opening passage of the book, which describes Montag at a burning house. It also describes his intense pleasure at burning. To understand the line you quoted, it helps to imagine a huge, burning house. The heat generated from a burning house, going up in flames, is going to be unbearable, feverish, and intense. To stand next to that kind of fire would be almost painful and would cause one to grimace; when someone grimaces, it often looks like a pained grin. So, if you are standing next to a fire that huge, it is going to be painful; the body's natural reaction is to grimace. The firemen grimaced at the heat of the fire. Bradbury called it a fierce grin (as a grimace often appears to be) because they were close enough to the flames for it to start to singe them; eyebrows perhaps starting to singe, the edges of clothings starting to singe. And it is when things start to singe that the fire drives the men back, away from it, because the heat is too much. So, they have a fierce grin from being close enough to the fire to be singed and driven back from it. Make sense?
Adding to the actual physics of the description is Bradbury's flair for descriptive writing. He picks his words carefully. For Montag, "it was a pleasure to burn," and he did it with immense joy. So to describe the closeness to the fire as a "fierce grin" instead of a "painful grimace" was a deliberate literary choice that Bradbury made. He wanted to convey the feeling that Montag, even though close enough to the fire for it to cause pain, did not feel pain, but rather intense pleasure. So, it was a fierce grin that Bradbury used.
I hope those explanations helped a bit! Good luck!