The entire interior of Serenity, and some of the exterior, was built full-scale and almost completely contiguous. It was split across two sound stages: one for the upper deck and one for the lower deck, shuttle docks, and hold.
The Alliance's full title is the "Anglo-Sino Alliance." Joss Whedon intended for it to be the merger of the USA and China, the last of the world's superpowers. That's why many characters sometimes speak Chinese. The Alliance flag, seen in the original pilot, is a blending of the US and Chinese flags.
Many of the names of off-camera and minor characters are drawn from the ranks of science fiction writers. Notably, Bester (Alfred Bester) as the original mechanic of Serenity and Brennert (Alan Brennert) and Ellison (Harlan Ellison).
The cast had a running gag where they would yell Summer Glau's name whenever they flubbed a line or messed up. It began after she forgot her line at the end of a particularly difficult scene. The gag continued through the filming of Serenity.
Some of the weapons used in the series were contemporary with the time of production and chosen based on their somewhat futuristic look. No modifications were made by the prop department to either disguise them or make them look more futuristic. Alliance soldiers are seen carrying British L85A2 rifles and Heckler & Koch MP5s, both in variant models. The Browncoats are seen using Heckler & Koch G36 rifles.
In several episodes (for example Firefly: The Train Job, Jayne wears a German police jacket, from the state of Rhineland-Palatinate (German: Rheinland-Pfalz). The badge on the right sleeve has the word "Polizei" (German for police) on the top and the emblem of Rhineland-Palatinate underneath.
Every scene in space is shot without sound effects of the ship moving, precisely as it would be in real life; no air, no sound. Of course, that doesn't cover the background music played during those scenes.
Morena Baccarin's character Inara is named for a goddess worshiped by the Hittite (Hurrian) Empire of what is now Turkey, circa 1800-1200 BC. Inara was the goddess of the wild animals of the steppe and daughter of the Storm-god Teshub/Tarhunt. She is analogous to the Greek Artemis and the Roman Diana.
Serenity's engine room includes the center console from a Boeing 737, complete with throttles, fuel cutoffs, spoiler and flap levers, and pitch trim wheels. It's frequently seen standing on the floor between the cot and the engine.
During the DVD commentary, it is mentioned that there was a subplot for Inara that was never developed but that was foreshadowed with a number of shots and/or lines during the series. At a 2008 Dragoncon panel, Morena Baccarin confirmed the long-standing rumor that that subplot would have involved Inara having a terminal illness.
Joss Whedon intended the show to have run for seven seasons. However due to low ratings, the FOX Network canceled it with only 11 episodes out of 14 airing on the network. Prior to it's cancellation fans formed a Firefly Immediate Assistance campaign which involves sending in postcards to the FOX Network to support the production of the show. After that didn't work, the campaign worked on getting another network such as UPN to pick up the series for a second season. The campaign ended up being unsuccessful in securing the continuation of the show.