You could use incandescent: Merriam-Webster defines it as:
Definition of incandescent (Entry 1 of 2)
1 a : white, glowing, or luminous with intense heat
b : strikingly bright, radiant, or clear
c: marked by brilliance especially of expression incandescent wit
d: characterized by glowing zeal : ARDENT incandescent affection
(see here). It’s also idiomatic to say
incandescent with rage
(see here) so would work well when describing somebody’s temper.
To expand on this, one common use of the word incandescent is in relation to light bulbs. Incandescent light bulbs comprise a wire filament (typically made of tungsten specifically because of its very high melting point) through which an electric current is passed in order to heat it to such a high temperature that it emits visible light. According to Wikipedia the filament reaches 2,000 to 3,300 K (3,140 to 5,480 °F), so potentially hotter than a phosphorous incendiary grenade. Admittedly, the bulb is usually filled with inert gas to prevent oxidation, so it is arguable that the filament does not "burn", but it certainly reaches very high temperatures.
My hesitation regarding incendiary is that it is often used in relation to starting a fire (see, for example, the first definition at Dictionary.com:
- used or adapted for setting property on fire
rather than relating to objects which necessarily burn at high temperatures. This is particularly true in its metaphorical uses (as in your example): an "incendiary comment" might be delivered coolly and calmly but is nonetheless incendiary in that it provokes, or is designed to provoke, a heated response.
As an afterthought, all this talk of fire makes me realise that another word to describe a hot-burning temper would be fiery:
- intensely hot
(Again from Dictionary.com).