Or drone bombers, if you prefer.
Those are enabled by nuclear weapons. Firebombing via drones would still be tremendously expensive.
Firebombing of European cities (constructed, generally, of much less flammable material than Japanese cities) took more work and was (thankfully) less successful. Dresden notably excepted.
Who's talking about the present situation? The OP was comparing the use of nuclear weapons against the Japanese to the use of conventional weapons against the Japanese in the same war. The bombing of Nagasaki and Hiroshima are sometimes treated as being qualitatively different from the firebombings of other cities because of the use of nuclear bombs and the OP was disagreeing with that sentiment. Obviously WWII era bombers don't compare with modern ICBMs but no one was saying otherwise.
The parents comment was that the suffering caused by the atomic bombs wasn’t that different than that caused by the fire bombing.
What do drone bombers have to do with that?
Costs matter. See the Jevons Paradox. And, as I've said before, scale matters (https://news.ycombinator.com/item?id=13542735#13548498) (and more: https://hn.algolia.com/?query=dredmorbius%20"scale%20matters...).
It matters more than very nearly anything else.
Seven planes took part in the bombing of Hiroshima, six in that of Nagasaki. That means that we can’t say those raids were less risky for US soldiers than conventional bombing.
Also “tremendous cost” applies to Hiroshima and Nagasaki, too, perhaps even more so.