We can assume that the time period for Firegirl is fairly recent if not contemporary. The book was published in 2006, so it's important to use context clues to try and determine when exactly the author, Tony Abbott, had in mind while writing.
Within the first few pages of Firegirl, the narrator describes the Cobra- a classic car from the 1960's. Based on the fact that he calls the car classic, we can assume the story has taken place after the 1960's, probably at least a few decades in order for the car to be considered "classic." This inference is also backed up by the fact that the Cobra he is talking about in particular, belonging to the uncle of his best friend, had to be restored. This implies the car had fallen into some degree of disrepair.
Another detail which helps us pinpoint the time period is the scene in which Jeff sets a toy car on fire. It is a remote-control Batmobile model made of plastic. Though model and toy car collecting has been a hobby since the first half of the twentieth century, the market was not catered to children until after the 1970's.
Based on the lack of mention of a cell-phone, and the fact that Tom uses a landline to call his friend, I would place this story sometime before it was common for most people to have a cell-phone- perhaps late 1990's or early 2000's.