Not being an expert as to the rules of war I wonder if civilians throwing firebombs gives an out for the attackers being called out for war crimes targeting civilians?

Not legitimately when the targeting of civilians comes first.
Sadly, I must admit America has used that excuse in nearly every military action I know of….targeting civilians and using any retaliation as pretext.

If you fight a war you have to use everything you have. The only limiting factor is own survival and post war public opinion, decided largely by who wins. There is nothing moral or legal about wars, every bet is all-in.

You only want to wage wars with clear objectives that at least your own population understands and that you know you will win, and be able to justify after the war is over.

Putin got something wrong. Or everything.

I do agree. However, I would have thought arming untrained civilians with napalm and (as I understand it) assault rifles reduces the chances of survival for them and their neighbours.

They are being indiscriminately bombed and shelled. This started before civilians took up arms en masse. I fail to see how fighting back makes their position worse…it seems to have done the opposite.
I believe NOT fighting back reduces the chances for survival for anyone who has supported democracy in Ukraine in the last decade. Putin will label them all terrorists and execute the lot of them if he’s not removed.