I actually read a book in highschool by a guy who was convinced that dragons were real, and backed up his case with a lot of good information. He also
used a lot of bad information, too, though, so it didn't really work for me. But his explanation of "dragon fire" was a valid one.
Many chemicals, when combined, react violently, causing flame. For example, if you put sodium into water, you get fire. This guy's theory was that
the dragon contained 2 chemical sacs in it's lower jaw, just like a spitting cobra's one. When the dragon would go to spew fire, these 2 chemicals
would squirt out, and meet a foot or 2 from the dragon's head, sparing the dragon from the heat of it's own blast. He believed it was a kind of
napalm type of fire, a gel that shot out which was covered in flame and would coat whatever it hit.
Not that I buy they ever existed, but if they did, that might have been how they did the fire thang.