There wasn’t much left to firebomb. By the middle of ‘45 more fliers were dying ferrying the planes to the Pacific than were lost over Japan. There was no resistance in the air.
That said, Okinawa proved what an invasion would look like.
Dropping the bomb was the “cleanest dirty shirt” in a drawer full of really dirty shirts.
The B-29 was arguably a death trap; there was a disproportionate loss of life among aircrews to engine failures and engine fires.
Had there not been a war raging (that could offer some justification for the incredible risks involved in serving on that plane), the B-29 would never have been allowed off the ground.