It depends on why the Motherboard cought fire , if from missuse then no. You have to make sure you are not at fault because on a RMA they stipulate that there be no visible damage. You will have to go through tech support anyway to get the RMA, because customer service wants to see a support ticket.
Exclusions for visible damage don't apply to manufacturing defects or damage created before you took delivery, and if you didn't short the motherboard to the case, then most likely there was a solder short, a faulty capacitor, or shorted MOSFET. Take photos before returning, with the motherboard on anti-static material (pink bubble wrap or bag) to prevent more damage and to prove you're a careful person.