What does the line "he strode in a swarm of fireflies" mean?
This is a figure of speech: it means the burning bits of paper, glowing red with the fire and floating through the air, are like fireflies. It's also a bit ironic, in that the figurative language is of great beauty, but the act it is describing is very ugly (the state-mandated burning of books). This whole opening section of the novel is designed to give some insight into the mental state of Montag. It is meant to explain the first line, "It was a special pleasure to see things eaten, to see things blackened and changed." How can a man whose work is to burn books love his job so much? The beauty of the fire is part of this; for Montag, the burning pages are beautiful because, through burning the books, he feels powerful: "With the brass nozzle in his fists, with this great python spitting its venomous kerosene upon the world, the blood pounded in his head, and his hands were the hands of some amazing conductor playing all the symphonies of blazing and burning to bring down the tatters and charcoal ruins of history."
In this excerpt, Bradbury is using a metaphor to compare the sparks and embers from burning books to fireflies. Montag, a fireman whose duty is to burn books, is walking through the aftermath of a fire, and these embers are fluttering around him as if he is striding through a field full of lightening bugs. Bradbury is an expert at using figures of speech like this to give his reader’s a visual picture of what is going on. There are so many metaphors and similes of fire throughout the book, and this is just one of them. Fire is used as a device in the novel to get across two opposite ideas. Although fire destroys, it is also a symbol of light and knowledge. In the scene where Montag is walking through the embers, he is really walking out of the dark and symbolically into the light and knowledge he later receives in the novel when he finds out that books need to be preserved and not destroyed.