Two Guys and Girl grad Nathan Fillion has been cast as the lead in Firefly, Joss Whedon's futuristic space Western series for Fox, according to The Hollywood Reporter. Firefly — which already has a 13-episode commitment from the network — is set 400-500 years in the future. In other TV casting news, Donnie Wahlberg has signed on to star in the NBC's cop pilot Boomtown.