Special Firebending techniques
Another type of firebending used in the show is lightning. Mentally, it involves a complete absence of emotion and peace of mind. Physically it requires separating the energies of yin and yang. When the forces collide, the bender only guides, rather than controls, the lightning's direction. By studying the techniques of waterbenders, Iroh developed a counter to lightning attacks. When attacked, it is possible to redirect the lightning through and out of the body. However, this technique is not without risk. It is important to route the lightning through the stomach to avoid shocking the heart and die. Azula and Ozai greatly feared this technique since they themselves don't know how to use it. Creating lighting appears to be a secret technique of the Fire Nation's royal family, as only Azula, Iroh, and Ozai are shown creating lightning. Lightning redirection is even more rare; by the end of the series Iroh, Zuko, and Aang are the only Firebenders to know the technique of redirection.
Some skilled Firebenders are able to project flames out of their feet, resulting in short boosts that enable them to jump much higher. During Sozin's Comet, when a Firebender is most powerful, this ability is greatly enhanced and they are able to sustain full flight for an unlimited amount of time.
Combustion Man (or "Sparky Sparky Boom Man") displayed a very unique form of Firebending, the ability to superheat air into a thin beam which results in large explosions. No other character has displayed this ability and it is unlikely that any other can, as the power appears to be connected to the eye-shaped tattoo on his forehead.
Part of the reason for Iroh's nickcame, "The Dragon of the West." Iroh can breath fire like a dragon with this skill.
Azula has the ability to bend blue fire, which has a much more intense heat. She is the only firebender that has shown this ability, not even Iroh, Ozai, or the Avatar has been shown to bend blue fire. This could argue that the only difference of blue fire is just the color. This was demonstrated when Zuko was able to deflect Azula's blast just as easily as regular fire. In the finale, Zuko had the upper hand in their duel, despite Azula's mental instability.