Trivia: Nathan Fillion, Gina Torres, Adam Baldwin, and Alan Tudyk all later played villains on Joss Whedon's other Mutant Enemy productions: Fillion appeared as Caleb in the final five episodes of "Buffy the Vampire Slayer," while Torres played Jasmine in season four of "Angel" and Baldwin played Marcus Hamilton in season five. Tudyk played Alpha in "Dollhouse".Cubs Fan
Trivia: When the crew returns from their raid of the Alliance hospital, Malcolm grabs Kaylee from behind and holds her in a friendly hug. This was improvised by Nathan Fillion, and was left in the shot because Joss Whedon felt it was such a natural movement for Malcolm to make. (00:37:00)Cubs Fan
Trivia: As Mal is bringing the shuttle in to land at the floating luxury home, as he flies to the left of a tower towards the gardens, behind them flying from right to left is the unmistakable silhouette of an SR-71 Blackbird cruising by. Given that this is a planet for the super rich and the heist is of a well known collector of old Earth artifacts, maybe this plane is part of another collector's collection, like John Travolta and his small fleet of planes including an airliner and other exotic aircraft.
Trivia: Nathan Fillion would hide snacks and drinks on-set (usually in crevices, drawers or just off camera), so they would be readily at hand in-between takes. After learning the series was canceled, he alleges that he purposely hid a half-finished drink in one of the pieces of the set, and never retrieved it, to see if anyone would eventually stumble onto it. He reportedly does not know what became of it, so there's a chance it's still in the set-piece in storage somewhere.
Trivia: Creator Joss Whedon, who is infamous for both killing off his characters and also having characters in relationships "break up", stated that if the series went ahead as planned, Wash and Zoe would have been the only couple in one of his series that would last from the start to the end, because he thought they were perfect together and shouldn't separate.
Trivia: If the show wasn't canceled, the contents of the mysterious syringe Inara has in the pilot would have been revealed. It was set to contain a very potent strand of an artificially created STD - non-fatal to Inara, but fatal to anyone who had sexual contact with her within a certain span of time after she injected it. In a planned storyline, Inara would inject herself with it shortly before being captured by a Reaver ship. After the crew of the Firefly breaks onto the Reaver ship to rescue her, they'd find every single Reaver dead, implying that Inara had been assaulted in some way and/or raped by every single one of them. This would then lead into an ongoing character-arc regarding her depression over the assault.
Trivia: Co-star Summer Glau reportedly flubbed a key line during filming one day at the end of a rather long take, and in frustration, one of the other cast-members (both annoyed but also amused by the flub) exclaimed "Summer!" Everyone on-set found this exclamation so funny, that they would all scream "Summer!" whenever they flubbed a line. Glau was reportedly annoyed by this at first, but later began to find it amusing like the others.
Trivia: Joss Whedon had intended for the series to last seven seasons, and had even come up with plans for the various main season-storylines before the series was canceled. The follow-up movie "Serenity" features a modified and shortened version of what would have been the central storyline for seasons two and three. In addition, "Serenity" also features most of the revelations about River's backstory and abilities that were planned for the rest of the series, such as her being trained to be a living weapon, her unknowingly having learned Alliance secrets, her knowledge of Miranda, her overt psychic abilities, etc.