Over at What-If, Randall Munroe considers the question about how many fireflies it would take to create as much light as the sun.
It turns out that the number, while huge, is finite: Three followed by 31 zeroes. Munroe simply figures out the output in lumens of the average firefly, compares it with the lumen output of the sun, and does a bit of basic arithmetic.
Not only is the number not outlandishly huge compared with some other numbers in the universe, it turns out that a firefly is brighter than an amount of the sun equal to it in weight. In other words, 20 milligrams of firefly is brighter than 20 milligrams of sun, although probably a lot easier to keep in a jar.
Of course, arranging the fireflies to produce the light output is a trick, since the outer ones would block the light of the inner ones if they were just in a clump. They would have to be in a hollow sphere with their heads facing inward -- which leads to the interesting situation of a sphere of moons as bright as the sun.
One way to get around the need to coordinate an immense hollow sphere of bug bums is to imagine one giant firefly with a luminous patch able to generate the light of the sun. According to Munroe's calculations, such a firefly would be as large as the solar system, but would have the unfortunate design flaw of instantaneously collapsing into a black hole. Not just any black hole, but a black hole larger than any black hole that has ever existed -- technically it would be too big for a black hole but if we're positing a firefly the size of a solar system we're already ignoring most of the laws of physics and so why not keep misbehaving?
Black holes evaporate very gradually in Hawking radiation, meaning that they eventually cease to exist. The "firefly black hole," larger than any other known, would evaporate last and would wind up being the last thing in the universe before everything decays into a random assortment of wandering photons, neutrinos, electrons, and positrons. Meaning that despite Gail Berman's best efforts, when everything else is gone, there would still be a little bit of firefly around.
Now that's shiny.