From Halopedia, the Halo wiki
Green, radioactive incendiary gel is used in the Mgalekgolo's assault cannons and in fuel rod weaponry. The Mgalekgolo's assault cannons are able to fire the gel in mortar-like globular projectiles or in a searing stream of fire, whereas the fuel rods are compressed canisters of the material which explode upon impact. A perfected form of the incendiary gel was known to the Forerunners, and was pumped around the bowels of The Ark for unknown purposes. In 2559, the Banished were able to take advantage of this Infusion gel in the form of their Grenadiers, Huragok and mines, among other nefarious purposes.
The Type-3 firebomb, a type of native Jiralhanae grenade, uses a form of adhesive incendiary gel; a stable yet hazardous chemical mixture containing caesium. It burns at approximately twenty-two hundred degrees Celsius, and will not burn underwater.