Source: The Sun

Learned From: Dragons

It has been revealed that Firebenders learned Firebending from the sacred dragons that populated their land. From them, they learned that Firebending represents harmony and life, not rage and anger as the modern Fire Nation would have its citizens believe.

Firebenders draw their powers from the sun and other solar objects. A solar eclipse has the potential to completely negate a Firebender's power, revealing a direct connection between the sun and Firebending. Additionally, while battling with Katara during The Siege of the North, Zuko stated that "you rise with the moon, I rise with the sun", further emphasizing the sun's importance to Firebending.