From a Joss Whedon interview conducted back in 2005
Could Serenity take place in the same universe as Buffy but the monsters were taken care of long ago?I think you could make an argument for it, but I wouldn’t. The only thing we ever thought of doing is having a drunken and very miserable…
THAT WOULD HAVE BEEN SUCH A CRAZY AWESOME WHEDON-FANGIRL MOMENT!!! ;___________;
WHY DIDN’T I GET THE CHANCE TO EXPERIENCE THIS!!!
Oh right… Fox.