Incendaries have been used since ancient times. Fire arrows were used by the Chinese and other people to set siege towers alight. The Byzantine Greeks developed a mixture of pitch and tar that was shot (pumped?) from shipboard urns. Called Greek Fire, dousing it with water would only disperse it and cause the first to spread.
In the Russo-Finnish War of 1939, the scarcity of anti-tank guns or rifles compelled the Finns to resort to glass bottled gasoline & soap mixtures which were and thrown onto tanks. When the glass shattered, the mixture would be catch fire from the lit rag that was tied around the bottle's neck. The "Molotov Cocktail," named in (derisive) honor of the Soviet Foreign Minister, was not the first time a hand thrown liquid incenary was used. Here's something from the War of the Rebellion (or Sybil Wa-oh) as we call it here in the USA.
"The sap-roller has been very much cut up by the enemy's fire, and was of no further use. I had just given directions to have it covered at once with earth, and to establish a trench cavalier at that point, when the enemy threw a fire-ball, which lodged under the edge of the sap-roller. They then threw hand-grenades into the fire made by the spreading of the inflammable fluid which it apparently contained; bursting, threw pieces all around it, tearing it considerably; at the same time they kept up an incessant fire of musketry on it. In about one-half hour it was entirely destroyed, exposing to their view a portion of the trench. The one on the right had been destroyed in a similar manner only an hour before.
Them Corn-feds were pretty clever boys.