Well logically speaking since some dinosaurs evolved into birds I would assume that dragons split off from this lineage, at least based off Occams razor (google it but short version is the simplest explanation is usually right). Therefore I would assume that the firebreathing and the large size (anything over the generally accepted max 25kg weight for flying birds) would make flight possible by using something like hydrogen (think Hindenberg airship). I would also use the bombardier beetle as to the mechanism as it can produce an explosive irritant at high temps as to how it would work.