The combat system of Avatar: The Last Airbender revolves around bending the four classical Greek elements of fire, water, earth and air, and each bending style is associated with a different nation and culture in the world, such as the detached Air Nomads and the tradition-bound people of the Earth Kingdom. Firebenders are another matter entirely.
Firebenders are unique because they can not only control flames around them but also generate fire from the vital energy of their bodies, making them more self-sufficient than other benders. All this comes at a cost though, since firebenders have few defensive options and can easily lose control of their element. Fire may be impossible to truly tame.
When Firebending Goes Out Of Control
In the original Avatar series, fire is associated with villainy, conquest and destruction, since nearly all known firebenders are soldiers or agents of the industrialized, conquest-hungry Fire Nation, from Prince Zuko himself to the ambitious Admiral Zhao and of course, Fire Lord Ozai himself. No bending element is inherently good or evil, but in the wrong hands, firebending can do far more harm than good, as opposed to the other three elements. Fire isn't evil, but it is a monster, and not even the best benders can truly tame it.
In the Book One episode titled "The Deserter," a former Fire Nation officer named Jeong Jeong explained all this to Aang, who wanted to learn firebending. As Jeong Jeong put it, fire is a living thing with an infinite appetite, and it will grow out of control and consume everything around it. By contrast, water, stones and air are relatively harmless, and stones won't move unless an earthbender bends them. Fire is never inert, which is the key difference setting it apart from water, air and earth. It takes on a life of its own, and firebenders end up more like monster tamers than artisans using a tool.
"The Deserter" made this clear when the action started and Admiral Zhao arrived to capture both Aang and Jeong Jeong. Zhao had once trained under Jeong Jeong, but he was impatient and reckless, and he never learned discipline or control. His firebending went wild in that episode, and Aang tricked him into burning down his own fleet of wooden ships. Zhao's fire threatened to consume the entire forest, and Zhao didn't care at all.
Similarly, in an earlier episode, Zuko arrived at Kyoshi Island and engaged the Kyoshi Warriors with his troops, including firebenders. The entire town was burned down, and Zuko felt no regret about the massive collateral damage. Invading waterbenders or earthbenders would have likewise caused collateral damage during the fight, but not to such an extent since dirt and water don't spread out of control the way fire does.
Later, firebending was used on an apocalyptic scale during the events of Sozin's Comet, when Ozai and his fellow firebenders used their comet-enhanced powers to launch the ultimate scorched-earth campaign and scour the Earth Kingdom's terrain of all life. By design, Ozai had no restraint, and he thought that fire could solve any problem put before him.
The Theoretical Limits Of Firebending
Avatar: The Last Airbender's combat system is well-balanced, and that includes fire's many weaknesses. Aside from Ozai's scorched-earth campaign, no firebender would want to burn down absolutely everything around them, and in many cases, sloppy or careless firebenders may be horrified at the unintentional damage they cause with out-of-control flames. Firebending is always risky, especially when a lot of flammable materials are found nearby, and it's better for a firebender to not bend than to go wild and cause unintended harm. Firebenders must contend with the natural limits of their element, and if they cannot, perhaps they shouldn't bend at all.
Notably, benders have limits on their bending "strength," and only with training and practice can they expand their skills. Katara couldn't casually lift a massive iceberg, nor could Toph stop a powerful earthquake all on her own. Similarly, episodes such as "The Deserter" make it clear that firebenders have limits as well; they cannot restrain or extinguish their flames once the fire spreads too far, such as if an entire forest is accidentally set ablaze.
Zhao might have meant to put out his fire during "The Deserter," but he didn't, likely because he couldn't control that much fire, especially since it was still growing while feeding on fresh kindling. Similarly, it's unlikely that Zuko could have easily put out the flames of Kyoshi Island's town after the fight even if he wanted to. At best, firebenders must wait for the flames to go out on their own in such situations, such as waiting for rainfall or for all flammable material to finally burn to ashes.
