There sure is a lot of fire and heat in this book. But like fire itself, it can be both good and bad: it warms people at night, but it also burns books and meadows. So, let's take a closer look.
In"Rocket Summer," fire shooting out of the rockets' engines warms up a cold Ohio winter and fires—excuse the pun—people's imaginations. On Mars, Ylla feels a great (metaphorical) warmth from the passing rocket that carries someone she might love ("Ylla," 176). Spender warms up the night with a wood fire instead of using more advanced technology ("—And The Moon Be Still As Bright," 1). Benjamin Driscoll has a fire as his only nighttime companion ("The Green Morning," 5).
Here, fire seems to be mostly associated with positive things—love and companionship and the natural world. It's warm both literally and metaphorically, and it acts as a little transportable symbol of the best things on Earth.
But fire is also dangerous and destructive. When rockets land in "The Locusts," the narrator says that they "set the bony meadows afire, turned rock to lava, turned wood to charcoal, transmitted water to steam, made sand and silica into green glass which lay like shattered mirrors reflecting the invasion, all about ("The Locusts," 1). So, humans haven't even started to settle yet, and they're already using fire to reshape (and destroy) the planet.
And fire destroys Earth, too. As people stand on Mars to watch the nuclear war, they see that "Earth changed in the black sky. It caught fire" ("The Off Season," 176-7). Obviously, this kind of fire is bad with a capital "B." This is humanity gone way off course. Not content with burning books ("Usher II"), they're burning themselves up.
And some instances of fire are a little harder to figure out. Take the dad burning all his Earth papers in "The Million-Year Picnic":
The fire leaped up to emphasize his talking. And then all the papers were gone except one. All the laws and beliefs of Earth were burnt into small hot ashes which soon would be carried off in a wind. (127)
It's hard to get a sense of what Bradbury thinks about this. On the one hand, sure, all the Earth laws and beliefs ended up leading to destruction. On the other hand—everything? Do we really want to burn indiscriminately? Surely there's something good left.
And maybe that's the problem with fire. There's no middle way: either it burns everything, or it burns nothing. Or—how about this? Fire is technology. In fact, you could argue that it's one of the first human technologies. And as "The Million-Year Picnic" teaches us, you have to be careful with technology:
[P]eople got lost in a mechanical wilderness, like children making over pretty things, gadgets, helicopters, rockets; emphasizing the wrong items, emphasizing machines instead of how to run the machines. Wars got bigger and bigger and finally killed Earth. (123)
So, whether it's fire or rockets, human technology is dangerous. Helpful? Sure. Amazing? Absolutely. But dangerous.