I'm not sure of exactly what you're trying to ask. I would say, "Where's the fire?" if I were asking the location of a fire. I might also ask this if I was wondering why someone was in such a hurry.

‘On fire’ describes the exceptional state of something. Your sleeve is on fire. I smell smoke. Something must be on fire. Forests are usually big and they have fires routinely and naturally. So forests ‘have’ fires but the forest itself isn’t typically ‘on’ fire. That would imply that the entire forest is burning.

A. Where is it on fire? Where is the fire at? B. *-in the forest-*

Depending on what you are trying to say... For example, if a house is on fire, and you want to know exactly where. You can say "Where is it on fire?" If you're trying to ask where the fire is, you would say "Where is the fire?"