Symbolism, Imagery, Allegory
You know that thing we were saying about the glass snowdrop as a symbol of love, which can be used to help or to harm? If you're not sure what we're talking about, then may we kindly suggest you read up on the pretty little flower elsewhere in this section. Because the thing is, this whole help-or-harm concept applies to fire in Stardust, too.
When Tristran and Yvaine are about to be slaughtered by the witch-queen, for instance, Tristran bravely thrusts his hand that is holding the candle into the fire to help them get the heck outta there. And while on the one hand, this results in super painful burns, on the other, it sure beats winding up dead. In just this one moment, then, we see fire manifesting both help and harm at once.
Septimus, for his part, decides to set a fire to entrap the witch-queen, who he (rightly) believes killed his brother Primus. This is a sneaky, underhanded way of offing someone. But instead of the fire destroying the witch-queen as Septimus sneakily intends, the fire brings about his own death. She is able to manipulate the situation to her own benefit, and though she's definitely not one of the good guys, again we see fire standing at the symbolic intersection of help and harm.
Because of this, on a larger level fire is a symbol for duality, insisting on acknowledgment of both sides of the same coin, if you will. So pay attention when you see fire—for every person who gets burned, either literally or metaphorically, there's likely someone who benefits, too.