system
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human
[Avatar: TLA/K] Where does the firebenders' fire come from?

Firebeding doesn't seem to be like the other bending arts. Airbenders can only bend the air around them, earthbenders the soil and ground beneath their feet/in the area, and waterbenders need to have water handy to bend.

Firebenders, on the other hand, conjure gouts of fire from their fists and feet. Since fire is the reaction of oxygen with flammable materials, where is their fire coming from?
gpt
All bending stems from a bender focusing and releasing their chi into their natural element through mental and physical exercises. For earth, water and airbenders, this makes nearby pieces of said element move or change depending on the bender's need. 

Firebenders are theorized to be most closely related to the ancient-upon-ancient energybenders of times long past. Their chi doesn't "move" fire so much as their chi fuels a reaction that can then be controlled. Like a flame thrower combining liquid fuel, an open spark and a source of pressure to launch its deadly payload far away. (Real life flamethrower, not a video game melee-range-only weapon) 

In this humble scholar's opinion, a firebender acts as the fuel source, spark and pressure all at once. When they perform a move, they draw from their sea of chi in the stomach, move the energy to their hand or foot, "spark" the chi through some means (probably another burst of energy, perhaps the breath. It has been said to be the most important and fundamental part of firebending after all) and then direct a stream of ignited energy out into the world. 

Fire is life, not just destruction and burning. So instead of moving an outside element, firebenders are working with their inner energy more than any other bender. It's also why a firebender with no drive or motivation has such a hard time producing a flame. With no "inner fire" to truly WANT to bend, the chi moves weakly and cannot spark.