Also Japanese construction was conveniently, for the US, extremely flammable so you could do way more damage starting a firestorm than you could with the same number of bombers filled with traditional bombs.
There was also the incredible plan to fill a bomb with bats strapped with tiny incendiary charges on timers so they would be dropped, go roost somewhere, and hopefully start even wider spread fires. They spent about 2 million dollars on it before it was cancelled because the atomic bomb was showing much better progress. They also accidentally proved it's effectiveness and burned down part of the testing facility.