Sorry, bad English.

To us now how would it be different if Hiroshima and Nagasaki had, instead, been firebombed ?

In terms of the destruction and civilian loss of life it was similar.

In terms of nuclear bombs they were and are primarily symbolic. The were tiny. Off the shelf deployed bombs are 400 times the size. The largest device ever exploded was over 2,000 times more powerful and could have been twice that. Nuclear Weapon yield