Sorry, by "also seen as war crimes", I don't mean to sound like a Wehraboo. My observations come from many people on Reddit saying that they were war crimes.
Firebombings of:
Tokyo
Hamburg
Dresden
Or was this a rare opinion because it was total war and because of:
Pearl Harbor
The Blitz
Coventry Blitz
Rotterdam Blitz
In a word, no. The prohibition on the delivery of incendiary weapons by aircraft over civilian areas was not added to the Geneva conventions until after the war. (1977 for Additional Protocol I, 1980s for Convention on Certain conventional Weapons).
More general protections for civilian population (as opposed to incendiary bombings) were put in for the fourth convention of 1949.
As a general rule, if nothing is specifically prohibited, it's legal. Thus, the bombings were considered under the extant general requirements of military necessity, and defended areas.