How do the characters Tom and Jeff change in Firegirl?

Although the narrator, Tom, changes significantly over the course of the novel, his best friend Jeff does not change--in fact, Jeff loses his chance to change, and the fact that Tom proves to be a dynamic (developing, changing) character while Jeff remains static helps illustrate one of the novel's main ideas. That is, when you have the chance to make friends with someone very different, or someone who's suffering, like Jessica in this story, you can either take advantage of that chance and grow from it (like Tom does) or you can let that opportunity pass you by and learn nothing as a result (like Jeff does).

Let's take a closer look at how Tom changes while Jeff remains the same.

Both boys start off being obsessed with classic sports cars from the sixties. They anxiously await the chance to ride in Jeff's uncle's Cobra, a snakelike machine that's all about speed and flash. But by the end of the novel, after Tom has befriended Jessica despite her disfiguring burns, Tom doesn't care much about that Cobra anymore. He chooses to spend time with Jessica instead, even when he finally gets the chance to ride in that incredible machine. Jeff, however, scoffs at Tom and his decision. Jeff doesn't understand that friendship and helping others in need may actually be more important than taking an exciting ride in an expensive car. Tom, however, does understand this.

Speaking of helping others, Tom also learns to become less selfish, while Jeff never learns this lesson. At the beginning of the story, when Tom's mother suggests that he go over to Jessica's house to befriend her, he wonders how that could possibly help him. You can just see Tom's mom roll her eyes at him when she tells him that it won't, but that it will help Jessica. At that point, it's as if Tom only cares about himself. But by the end of the story, after making friends with Jessica and listening to her as she works through her grief and pain, Tom learns to sacrifice his own comfort and his free time (and his time with his best friend Jeff) to help Jessica. He understands that helping others is worthwhile, and the right thing to do.