It looks like Joss Whedon was unable to let go of the world of Alien: Resurrection, for which he wrote the screenplay; a little surprise left in the first episode of Firefly implies that he placed his cowboys and aliens in the same universe.
When Mal’s ass is getting kicked at the Battle of Serenity Valley and he hops into an anti-aircraft cannon to blast down an Alliance fighter. The heads-up logo that comes up when he powers it on includes an interesting logo in the center.
Check this out:
See the resemblance? Including the Weyland-Yutani Corporation logo says something interesting about the worlds that we once thought were separate.
Looks like Mal and his crew could easily have had some close encounters with Ripley.