Firefly MMO? Maybe?
Should you not want to wait, there is online Firefly roleplay in Second Life. Our sister publication, the ‘Verse Voice, covers much of the RP news, along with some general Second Life and science fiction news. You can see the oldest of the Firefly SL communities here in Washtown (the crappy little town named after Wash the pilot. It also has information of Firefly in SL and the other Firefly and those that just work with the roleplay. You will have to join Second Life to visit, but you can do that for free.