Deepawali, as the name itself suggests was marked by the lighting of lamps on the dark night (new moon) of karthik or the Karthik Amavasya. It is the darkest night of the year. The lighting of lamps therefore became a symbol of hope amidst the darkest night, of the victory of light over darkness. Hence the goddess of the full moon, Lakshmi, and the goddess of the darkest night, Kali came to be worshipped on this day, one as the destroyer of the forces of darkness (kali), the other as a source of fullness even in the state of poverty, of night.
Later stories got associated such as the return of Lord Rama to Ayodhya after victory over Ravana on Dussehra day, the victory of Sri Krishna over Narakasura on the day previous to Deepawali. All these stories are clearly beautifully symbolic.
Firecrackers came much later with the gunpowder. Later the Chinese who sold it for celebration purposes. It has nothing to do with Deepawali nor has it any significance though human mind can always device significance behind things. It is pure business, nothing else.
Affectionately,
Alok Da
