Ukraine, well they don't have nukes but as I understand it, much of the USSR's nuclear arsenal was built in Ukraine. I suspect Ukraine could respond tit for tat.
But it would and always has been the last resort. If Russia feels like they have no choice left, they might do it.
But also, at the start of the war they used it as a deterrent, promising to use them if e.g. Ukraine were to strike across the border. That ended up being a false threat in the end, but you can see how Ukraine only slowly and carefully started becoming more and more bold with going across the border. All bets are off now though, with long range drones being used to target the very vulnerable refineries and oil industry. If they take out the power industry as well, and given time, it'll collapse Russia's military logistics network and isolate the front lines from supplies.
Buildings in Kiev aren't made out of wood. Firebombs would do very little damage.
Russia hasn’t captured Kiev, their artillery can’t reach that far and they don’t have air superiority - Russia hardly has an airforce anymore.
They toss bomb from miles back.
Flying conventional bombers over enemy cities requires the ability to replace most of your bombers per year, or air supremacy, neither of which Russia has even a hope of doing.
WW2 was industrialized the likes of which nobody has ever seen again.
Spending untold billions of dollars and tens of thousands of lives for a pile of ash would never pay itself off.