What occasions are you referring to? I know people claim that Israeli use of white phosphorous munitions is illegal, but the law is actually quite specific about what an incendiary weapon is. Incendiary effects caused by weapons that were not designed with the specific purpose of causing incendiary effects are not prohibited. (As far as I can tell, even the deliberate use of such weapons in order to cause incendiary effects is allowed.) This is extremely permissive, because no reasonable country would actually agree not to use a weapon that it considered effective. Something like the firebombing of Dresden is banned, but little else.

Incendiary weapons do not include:

(i) Munitions which may have incidental incendiary effects, such as illuminants, tracers, smoke or signalling systems;

(ii) Munitions designed to combine penetration, blast or fragmentation effects with an additional incendiary effect, such as armour-piercing projectiles, fragmentation shells, explosive bombs and similar combined-effects munitions in which the incendiary effect is not specifically designed to cause burn injury to persons, but to be used against military objectives, such as armoured vehicles, aircraft and installations or facilities.

The moral high ground doesn't work in war.

The moral high ground is often the losing low ground, unfortunately. I’d say Ukraine should stick to the rules of war (as should Russia) and we should remove all restrictions we place on our donations to Ukraine - and enforce a no-fly zone over western Ukraine, at Ukraine’s invitation. There is only one way to make Russia stop and that’s force.

Why is it even morally reprehensible? If you you blow the guts out and faces off Russian soldiers by more traditional means they are just as dead and if dozens of Ukrainians die in the course of digging the Russians out of cover do you account that a superior outcome? If so how?

If a burglar strode into your home with a gun and you believed that conflict was inevitable how much risk and or suffering would you tolerate from your wife and children in order to decrease the chance of harm or suffering by the burglar? Would you accept a 3% chance of a dead kid in order to harm instead of kill the burglar? Would you take a 1% in order to decrease his suffering substantially?

My accounting is that there is no amount of risk or harm I would accept for me and mine to preserve the burglar's life because he made his choice when he chose to harm me and mine. I wouldn't risk a broken finger to preserve his entire life nor should I. That said should he surrender I would turn him over to the police. I should never take opportunity to hurt him let alone execute him. Should I do this I would be the villain no matter what had transpired before because I would be doing so out of emotional reaction I wouldn't be acting any longer to preserve me or mine.

We should expect Ukrainians to take any possible advantage for in doing so they preserve innocent life. Preserving the lifes or preventing the suffering of active enemies presently actively trying to do harm is nonsensical.

The United States and the UK successfully blocked attempts to outlaw all use of incendiary weapons

That's because incendiary weapons are great for exterminating villages full of poor people in the colonized world - ie, the kind of wars the US and UK prefer to wage.

Ahh, so wound them just enough is the optimal amount of mangling

Battle of Stalingrad II: Ukrainian Boogaloo

It's because of their indiscriminate nature.

The US use of napalm on cities in Korea contributed to the nearly 20% of their population that was wiped out.

Preface: I am no expert, this is just my understanding.
Weapons that are illegal/considered war crimes fall roughly into categories of:

A. Indiscriminate - kill soldiers and non-combatants/civilians alike (eg. Land mines, incendiary, cluster bombs, etc)

B. Cruel - especially painful ways to die or designed to cause ongoing suffering and maiming. (Eg: gas/chemical warfare, dirty bombs, etc)
A lot of weapons tick both of those boxes, and there are possibly more i am unaware of.

Yes

I assure you one thing: If it happened to you and you survived, you will not wish this on your worst enemy.

i have a hard time explaining this to people, they simply don't get it-.