"The only way to get to an air temple is on a flying bison, and I doubt the fire nation has any of those!" - Aang
Fire Nation "Air Power"
The concept of fire nation "Air power" doesn't come up until Season 1, Episode 17 "The Northern Air Temple". This is the first creation of the war balloon, proving that it was not around during the initial air temple invasions 100 years prior. The war tanks with the grappling hooks are also implied to have been created later, as it is mentioned "I remember my dad tinkering with the counter-balancing system", by Teo. For the fire nation zeppelins, those are much larger and heavier than even the war balloons and might have come after that, considering the zeppelins don't show up until later in the series.
Dragons and Fire Power
Now, at the time of Roku's death, dragons were still around. Firelord Sozin did start the idea of hunting them, but the dragons might have been around during the invasion of the air temples. However, the dragons are not a solid theory on this. Only Roku and Firelord Sozin are shown with dragons, and they are both powerful men within the show; it's doubtful just anyone could train a dragon to be theirs, or used to carry men to an air temple.
The first air temple invasions took place during the first Sozin's comet (though later named), and then the theory of "Irohman" comes into play. During the second coming of the comet in the third season finale and retaking of Ba Sing Sae, the watcher sees Master Jeong Jeong "flying" as he uses fire from his feet to hover in the air. Firelord Ozai also uses the same tatic during his fight with Aang. It's possible the fire benders could have propelled themselves up the mountains this way.
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