A bombardier beetle creates a toxic spray by mixing two fluids into a single orifice. The fluids are chemically relatively inert when kept separated but produce a lot of heat when mixed. With only a bit of imagination, you could have a human producing a similar pair of chemicals that spontaneously ignite when mixed. Highly modified salivary ducts and a somewhat fire-proof mouth lining could then let them spit fire.
But I can't imagine it would be terribly efficient. Maybe good enough to light a cigarette, or give someone a fright, but hardly suitable for incinerating enemies.
One issue would be volume of fluid. A salivary duct produces only fractions of a ml per second, so in order to produce a significant volume of 'fire' the fire-starter would need some sort of internal bladders to store the stuff until they were ready to spit. Maybe mammary ducts would be more appropriate in that case...