The pigment luciferin is responsible for the bioluminescence of fireflies.  This pigment is oxidize

The pigment luciferin is responsible for the bioluminescence of fireflies.  This pigment is oxidized by a certain enzyme, resulting in the release of excess energy in the form of light.  The efficiency of this light radiation in fireflies is extremely high and ranges from 87% to 98%. For comparison, in an incandescent lamp, only 5% of the energy goes into the light, the rest goes into heat.