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.