Fireflies, like all insects, don't breathe. They use diffusion to get oxygen to their cells. Ergo, all their breathing requirements (0%) are met in full in any environment. Also, I do not have to explain how fireflies get into space to prove they don't breathe. That's like you asking me to explain how fish got on Jupiter to understand how gills work. Preposterous.
http://en.wikipedia.org/wiki/Diffusion
That's not what I was asking you to do. I know they don't breath like we do, but they "breath" air. Not with lungs. The air goes into the tracheal tubes and passes through the system of branching trachea. The circulatory system of insects, like that of other arthropods, is open: the heart pumps the hemolymph through arteries to open spaces surrounding the organs; when the heart relaxes, the hemolymph seeps back into the heart.
Still, show how there is enough oxygen in space, and how they will survive a trip to space, and let us not forget, how they will survive in an almost 100% vacuum environment. In space, there are high-energy protons and electrons, ultraviolet radiation, atomic oxygen, high and low temperature extremes, hard vacuum, galactic cosmic radiation, micrometeors, man-made debris and a lot of other things as well.