Also, think of it from the F.N.'s side. In midevil times, if you went against the lord or whatever you were killed. There were probably protestors back at the start, but I have a feeling they were made 'examples' of. Think public hangings but Fire-Style.
And the war has, as the other 2/3 nations complain about, torn families apart. How many Fire Nation wives, sisiters, children, etc. had to watch their loved ones be, quite likely, dragged away because if they said no there'd be some "If You Don't Join??The Army??Your Family Will Pay" thing.
And the propaganda that anyone in a dictatorship deals with. Whole generations have been fed the same garbage, like what Zuko was monologing about in SotNP. There really isn't much else they can base their ideas on since the only interaction the F.N. has with the other nations??is to attack them.
All Fire Nation people probably think their own way about the war, just look at Jeong-Jeong. I just think Ozai and the other Fire Lords have oppressed them and kept them in the dark for so long they don't really know better.