Cover art for Buffy Season 9: Spike #1

Buffy: “What are you doing here? Five words or less.”

Spike:  ”Out. For. A. Walk. Bitch.”

Angel: “We don’t have a…”

Spike: “A name? Well, that’s probably for the best. You’d want to be Angel’s Avengers or something.”

Spike in Firefly Season 6

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  Spike at a bar just sitting there going, ‘Nothing changes. Trust me, nothing changes.’  That would’ve been in our decadent era. That would have been like season six. But no, I have to think about them as totally separate  because they have different sensibilities and they have different sets  of rules. It’s very important that Serenity be grounded. I had to go  into outer space to come back down to earth but Buffy was a fantasy, a  metaphor for rites of passage of our lives and Serenity is something else.
“You were my sire, man! You were my… Yoda!”
Spike