Great question!
First, let's think about HOW the firefly emits light and then we'll talk about WHY they do it.

Scientists refer to this kind of light emission by an organism as a type of bioluminescence - the production of light through a chemical reaction in a living organism.

The firefly (and some other, related animals) produce a compound called luciferin. They also produce an enzyme - called luciferase - that can oxidize luciferin (in the presence of oxygen and ATP, an energy source), which results in the emission of light of a particular wavelength.

Interestingly, slight differences in the luciferase in different species result in a different wavelength of light emission of the luciferin. So, different species can emit different wavelengths of light. The luciferin and luciferase are produced in localized tissues, such as the ventral organ. The Wiki site on bioluminescence is an up to date, scientifically accurate place to start to learn about this process:

Google images also can lead you to some astounding examples of bioluminescence in nature. Many marine organisms exhibit various forms of bioluminescence as well.

But how does the firefly control the light emission and why do they do it? If you have watched the beautiful display of fireflies flickering over a field or along a fence line on a summer's night, you will notice that they have regular patterns - it's not random.

We don't really understand the exact mechanism of how the firefly controls light production (that is, how the luciferase activity is controlled). However, it is pretty well established that they use luminescence as a mating strategy. In some cases, the females emit light to attract mates. In other species, it's the males that emit light in an effort to attract females.

Once you start looking into the amazing world of bioluminescence, it's hard to stop!