Alchemist’s Fire is basically the Molotov Cocktail of 5e, so I’d just use that.

1d4 Fire damage per round unless they take an action to put the fire out seems pretty reasonable to me. Puts it on par with a shortsword at the very least.

Fireball’s damage is insane (the designers intentionally made it deal more damage than spells of the same level “because it’s an iconic spell”), so I wouldn’t really use it as a baseline for balancing anything, personally.