The Firecracker is an explosive gadget, that is available for 10 for 10 Firecrackers. It was introduced during the Chinese New Year update/event. It is actually a temporarily re-skinned version of the Dynamite. It could be acquired by completing tiers in the Chinese New Year Events.
This powerful gadget is pretty much a re-named Dynamite. Just use this the same way as the said gadget.