One might easily imagine the biological synthesis of a compound that ignites spontaneously when ejected forcefully into the air. One candidate might be the organic solvent diethyl ether. This is a colorless but sweet-smelling liquid that produces a copious, highly flammable vapor.
Once in contact with air, diethyl ether does not need to get into a flame to start burning fiercely: All it needs is a warm stovepipe, or even the static electricity generated by moving the liquid from one vessel into another. A dragon could keep reservoirs of liquid ether tightly bound in airtight sacs, like venom glands in snakes, evolved from salivary glands.
Forcible, muscular movement of the liquid through the blood-hot mouth cavity and across the teeth would be sufficient for ignition and the production of an impressive yellow-orange flame. Because diethyl ether produces so much vapor, a little of it can go a long way.
At left, a Western Diamondback Rattlesnake (Crotalus atrox) striking.