The Firefly "'verse" never made sense to me either. Whedon admitted he never put too much thought into it, aside from just having one system with as many planets and moons as he needed to make his stories.
Fans have posited that the 'verse is actually a quinary system with FIVE stars to worry about
I still wouldn't buy it as a viable place, but hey - in 2003 we only knew of a handful of planets period, and now we have hundreds with literally millions more out there. What the hell, it's good sci-fi.