Character design is one of the key aspects of animation, and Avatar: The Last Airbender excels at it. With four world powers at play, each Nation needed a distinct and recognizable sense of fashion in order to separate them from the others at a glance. Avatar went above and beyond by not only making the clothing of each nation unique, but by having it reveal key aspects of the Nation, if not the individual, who wears it as well.
From military armor, to commoner school-clothes, one thread can be found connecting all of Fire Nation fashion, uniformity -- and not just in the uniforms. Almost every citizen, military and civilian alike, dress in the Fire Nation's color palette of red, gold and black, showcasing not only their nationality, but ideology as well. As Zuko reveals, children are taught that war is a way of sharing their greatness with the world. In that regard, the people see themselves as lights in the darkness of ignorance (represented by the color black,) bringing progress and prosperity (represented by red and gold -- the colors of fire and wealth.)
Ironically, these colors are just as easily skewed -- much like firebending itself -- into colors of anger and oppression, something many corrupt Fire Nation higher-ups demonstrate over the series.
It's fitting, then, that it's mainly military personnel who wear black, as they are "going out into the darkness" by fighting the actual war -- in contrast, normal everyday citizens wear duller reds that are almost pink or brown, signifying their disconnect from the war. However, a vibrant red regardless represents strength, which is why it's so common among youth, the Gaang and the royals.
Despite the Fire Nation's emphasis on presenting a national color palette, their clothing also takes the individual into account, as there are wildly different styles of apparel available for those not in the military that consider body type, profession and personal preference. With the Fire Nation's unsurprisingly warm atmosphere, an emphasis on loose breathable clothing that exposes a fair amount of skin has pervaded the non-militarized land.
Likewise, the signature shoulder-pads of the Fire Nation soldier are a combination of beautiful formal attire and protective armor designed to strike fear into the hearts of their opponents. This showcases the Fire Nation's unyielding and uncompromising nature as a whole, as both function and form are given full attention. If you're going to take over the world, you may as well do it in style.
The Fire Nation's various uniforms emphasize this even further by using fashions to augment and improve their defenses; for example, the bending uniform's short chest-plate is designed to be visually appealing, projecting strength by outlining the pectoral muscles, while simultaneously deflecting blows away from vital organs and allowing benders unrestricted movement -- something that Azula takes full advantage of.
Likewise, the non-bending uniform focuses instead on protection by giving them full chest plates, and what appears to be chain mail. The subdued design is yet another protective measure, as it makes the non-benders less noticeable on the battlefield. In other words, the Fire Nation has outfitted each of its soldiers with clothing that allows them to make the best use of their abilities, which has the side effect of proclaiming that each soldier is more than just a military pawn -- but a valued member of society.
Another notable aspect of the uniforms is the similarities and dissimilarities between genders. As Princess Azula and her elite team prove, the women of the Fire Nation are seen as just as capable as their male counterparts and fully expected to take on as much responsibility -- but they don't have to give up their femininity to do so. This is best exemplified in the Boiling Rock guard uniforms; the bare midriff appears to be a fashion trend among the Fire Nation's women, and rather than deny their guards the chance to participate, the Fire Nation took full advantage of the increased mobility and cooling properties to incorporate it into the uniform.
The uniforms even fit each individual perfectly, suggesting they're individually tailored to prevent ill-fitting clothes from hampering their movements. Thus, by catering to the individual, the Fire Nation is able to present an even more united front with loyal military personnel who feel that their needs are being met.
And so, simply by examining the various character designs, we can see that the Fire Nation is an uncompromising society and military force that both genuinely cares about its people and holds to national ideals while being rightly feared by those whose lands they invade.