Jar of Fireflies is an item that can be purchased in the first catalog, Chimney Stuffer. It cost 10 Tomorrow Bucks. It is also the first item you send to Sugar Plumps and one of the 4 items used for the End Combo.
The jar will break if too much heat or too much force is exerted on it, leaving behind the lid (which is burnable, and burns for a rather long time). When the jar is broken, the fireflies will fly out, and the strange face inside the fireplace will open its mouth, letting the fireflies in.
"You won't want to open it in the morning."