There are several theories about breathing fire. One suggest a specialized bladdar storing certain chemicals and when exhaled, sprays a highly flamable venom. Another theory suggests iorn and other useful minerals stored within their mouths, and a specialized bladar storing methane that generates overtime as the Dragon pumps some adrenalin, and in a very complicated series of sentances I don't feel like saying right now, the Dragon would then exhale and all the flamable substances will combine with the methane and oxygen gasses and will then combust. It's a stretch, but there are many things about life that are truely paranormal, such as the Angler Fish's ability to glow in the dark.