Firebombing would also fail to intimidate the SOVIET command. I've frequently read postulations that was an additional impetus for dropping the 2nd bomb: to demonstrate to Stalin that we could mass produce them, lest he try to make any more aggressive moves in Europe. It was also a convenient way for the Japanese Emperor to "save face". People also postulate that the overwhelmingly successful Soviet invasion of Manchuria had the Japanese leadership worried: the Soviets had a reputation for brutality and a possible Soviet invasion of the Home Islands would turn out even worse than an American one (for the Japanese people). It looks much better to say "we threw in the towel due to America's wonder-weapon" than "we threw in the towel because we were terrified of Stalin".

Japan was already preparing to surrender before the bombs were dropped.