Fire can be both a friend and a foe, depending on where you're coming from.
Dustfinger—who can do all sorts of cool tricks with fire—misses the way it responded to him in the Inkheart world. There flames:
[…] had danced when he said the word. The flames here were both tame and mutinous, strange, silent beasts that sometimes bit the hand that fed them. Only occasionally, on cold nights when there was nothing but the flames to stave off his loneliness, did he think he heard them calling to him, but they whispered words he didn't understand. (31.8)
So flames represent both Dustfinger's nostalgia for the good old days, and his ambivalence about being in our world, where fire is an inert force rather than an active friend.
And yet fire is also what drew Capricorn into Dustfinger's life in the first place:
[…] it was also the reason why Capricorn had summoned him back in that other life. "Show me how to play with fire!" he had said when his men dragged Dustfinger before him, and Dustfinger had obeyed. (31.9)
So it's sadly ironic that fire, which Dustfinger thinks of as his friend, is also what brings him to Capricorn's attention.
Speaking of Capricorn, fire is a tool of destruction for this dude. Check it out:
Capricorn loved to give fire free rein, catching it again only when it had eaten its fill of crops and stables, houses and anything that couldn't run fast enough. (31.9)
Yeah, Capricorn sounds like a pyro in the worst way. His relationship to it makes it abundantly clear that this guy's main investment is destruction, an investment that stands in stark contrast to Dustfinger's almost friend-like relationship with flames.
So we see fire as multifaceted in Inkheart, just as it is in the real world—it can entertain and warm, or it can destroy and devour. We're thinking that fire works as something like a mirror, meaning that you'll see in it whatever is already within yourself. Capricorn's a destructive dude, so when he looks at fire he sees destruction, while Dustfinger is a resourceful entertainer, so he looks at fire and sees the potential for delight.