Although this isn't the last episode chronologically, The Message was the last Firefly episode made on set -- watch that last scene in the snow and you'll see the last scene that the cast was together. That alone makes it worth watching, but of course there's more good stuff going on than just OOC historical info.
An old war comrade of Mal's and ZoŽ's named Tracey smuggles himself onboard Serenity cunningly disguised as a dead man. Turns out he's smuggling valuable enhanced organs inside him. Other folks want those organs back. This leads to a heap of trouble.
Along with Heart of Gold, this episode was nominated for a Hugo award for Best Dramatic Presentation (Short Form) in 2004.
Simon: Yep, it's a cow fetus.
Wash: Oh my God, it's grotesque! Oh, and there's something in a jar.
Simon: This may come as a shock, but I'm actually not very good at talking to girls.
Zoe: Why, is there someone you are good at talking to?
Wash: Do not fear me! Ours is a peaceful race, and we must live in harmony.
River: My food is problematic.
Mal: He calls back, you keep him occupied.
Wash: What do I do, shadow puppets?
Nee GAO-soo NA niou, TA yo shwong mei-moo?
shiong tse sha sho
Wuo duh MA
For more information and a pronunciation guide, see the Firefly Chinese Pinyinary(approve sites).
Tracey was played by Jonathan M. Woodward(approve sites), a Whedonverse repeat offender(approve sites). In addition to his role as Tracey, he appeared as the amateur psychoanalyst/new vampire Holden Webster(approve sites) in the Hugo-winning Buffy(approve sites) episode Conversations with Dead People(approve sites), and had a recurring role on Angel(approve sites) as the Wolfram and Hart(approve sites) scientist Knox(approve sites). (The WhedonWiki(approve sites) link for Knox is a bit out of date; his character was more pivotal in Angel's final season than the page lets on.)
According to Tudyk and Staite on the DVD commentary, the only other actor to have a Whedon "hat trick" is Carlos Jacott(approve sites), though his roles were more minor. Jacott was Lawrence Dobson in Serenity, Ken in the Buffy(approve sites) episode Anne, and Richard in the Angel(approve sites) episode The Bachelor Party(approve sites).
This isn't actually correct, though, as they overlook Andy Umberger(approve sites), who had a recurring role on Buffy(approve sites) as D'Hoffryn(approve sites). On Firefly, Umberger surfaced as the Alliance Cruiser Captain in the pilot episode; on Angel(approve sites), he was the bad guy in I Fall To Pieces(approve sites).