Taiping Rebellion? I did an essay on that in year 12. Since then, I've cringed every time a textbook tries to tell me that 'World War I was the deadliest conflict yet'. Sure, if 10 million is a higher number than 20 million...
Didn't the shot in 'The Avatar and the Fire Lord' of Sozin overseeing the Fire Nation army include metal ships? I was fairly sure it did. Tanks were possibly invented during the war. Submarines and airships certainly were.
As a side note, I think we can easily say that the Fire Nation was relatively advanced because of their ready access to firebenders for industry.
I think the main reason guns might not have been invented was that bending already filled that role (accurately-firing medium-range weapon). Bending also filled the role of artillery plenty of times as well, as clearly seen in 'The Earth King' when Appa is attacked by surface-to-air earthbending emplacements. The potential for firearms was certainly there, given the presence of gunpowder and other explosives.
Interestingly, those surface-to-air emplacements in Ba Sing Se were built before
the Fire Nation army gained flight capabilities. To me, this means that they were designed to counter the Air Nomads in some prior conflict (or at the very least a situation with the potential for such a conflict), as the Air Nomads were the only force that could conceivably fly.
Unless Xin the Conqueror trained his own sky bison armada...