What does a firefly use its light for?
To find and attract a mate.
Firefly gets it's light by what is called as bioluminescence. In this phenomena, you get the light generated by animal, plant or some types of microorganisms. Here you get the light generated without production of the heat. In case of firefly you get the light intermittently. So that the the partner should be spotted, when light is on and the risk of being caught by the predator is reduced, when the light is off.
If the sequence ATG codon in a specific firefly should become ATA after a mutation will the firefly be able to emit light?
yes. the fireflies light is an important part of the fireflies body and removing it will not kill it instantly but it will disable the insect from flying and walking. there is also a chance of pulling the fireflies stomach organ out with the light however the stomach is to small to see with the naked eye. it will hurt and eventually kill the firefly if you remove the light from its abdomen