Margaret Weis. And one of the writers is Tracy Hickman. You may know this writer duo from the AD&D Dragonlance Chronicles, published in the 80-ies.
Firefly, or rather the Serenity RPG, is an over 200 page stand alone set of rules based on the Cortex engine. That's the system used in all Weis products, where you can throw bigger dice if your skill is better. It looks solid, thorough, hardcover - and slightly too full of layout artwork like most current RPGs. But it also does look like the real thing.
The content listing is exhaustive enough - with an emphasis on playing rather than rules. Which is an approach I like - less rules, more play. The main heroes of the series are also provided as characters, if you wish to use them. And there is an appendix on Gorram Chinese. That's the language that's used to swear in in the Firefly 'verse. Very convenient if you don't want to be rude in English!
The game and its expansions are available as pdf's (to read on your e-book reader or pc) here for example, on DriveThruRPG. Or you may try to order the more expensive hardcover at the main site. By the time you read this I may have bought the game, so if you don't hold your breath too long, a review may follow.