We don't have anything that specific about Truman's thoughts on the bomb before they were used. He was told it would be a tremendous bomb. He was probably under the misconception that Hiroshima was mostly populated by people in the military. When he learned of the actual situation on the ground there his reaction was to become concerned about killing so many innocents.

Personally, I would not trust anything Truman said or wrote after the end of the war on this issue, because he, like so many others involved in this, quickly re-wrote history in a way that made him seem incredibly competent and on top of things — almost prescient — in order to avoid criticism (of which there was heaps). Every scrap of evidence we have from the period before the bombs were thought to have "worked" points to a much more chaotic and confused understanding of what was going on. (On this, see Michael Gordin's Five Days in August). In any case, he does not compare the atomic bombings to the firebombings in his Memoirs — he doesn't really defend it much. I am unclear how much of his own memoirs he actually wrote (when I read them, it looks like someone else has just strung together a lot of archival files in his voice — his own opinions seem largely muted, if he had any).

As for the firebomb/nuclear bomb comparison, it is true that many of the people justified the use of the bombs as part and parcel of the existing firebombing policy that had already been used to destroy at least 67 other Japanese cities, and kill over a hundred thousand people in the March 1945 Tokyo firebombing raid. There are aspects of an atomic bombing that can be reasonably asserted to be similar (morally and phenomenologically) similar to a firebomb. There are aspects which are different.