UPDATE: Here are a few more suggestions supplied by Dragonlaird Gaming Fan, Theresa.
Thanks, Theresa!
Back before the movie was announced, before the TV soundtrack was released, I put together three CDs of music I thought was Firefly-esque. I find it fascinating that in looking at a couple other lists like this that others have posted, there isn't any overlap. Firefly has some touchstones musically (western guitar, soaring solo violin) but can really adopt many different flavors. Different moods can be set with music that is more modern (core worlds) vs. more throwback to Civil War era (the Rim). Action sequences can be punctuated by judicious use of electric guitars. All sort of possibilities. I hope this list is helpful to your Serenity game.
NOTE: Any tracks not attributed were dubbed off of the DVD set of the Firefly TV series. Most of these have been made available in better forms on the official TV series soundtrack.
Firefly Theme Song (with vocals)
A Unification Bar
Aberdeen - David Goodrich
Inara is Leaving
Hunger - Blackhawk Down Soundtrack
Love Scene
Iguazu - HBO's Deadwood Soundtrack
Wayfaring Stranger - Cold Mountain Soundtrack
Loved I not Honor More - Last of the Mohicans Soundtrack
Arriving in Deadwood - HBO's Deadwood Soundtrack
Leave No Man Behind - Blackhawk Down Soundtrack
Still - Blackhawk Down Soundtrack
Heart of Gold Segue
Smoke N'Oakum - Master and Commander Soundtrack
I Wish My Baby Was Born - Cold Mountain Soundtrack
Into the Fog - Master and Commander Soundtrack
Wounded Button - Open Range Soundtrack
Card Game - Open Range Soundtrack
In Time - Punisher Soundtrack
Last of the Mohicans - Last of the Mohicans Soundtrack
Face Off - Open Range Soundtrack
Laudanum Dream - Open Range Soundtrack
Chase Into Atmosphere
Hovercraft Approach
Still Reprise - Blackhawk Down Soundtrack
The Man's Too Strong - Dire Straits
Revenge - Legends of the Fall Soundtrack
Firefly Theme Song (end credits, no vocal)
Vale of Plenty - Blackhawk Down Soundtrack
Ashokan Farewell - Ken Burns' Civil War Soundtrack
Ashes to Ashes - Blackhawk Down Soundtrack
Of the Earth - Blackhawk Down Soundtrack
Neil Gows Lament - Pete Clark
DVD Montage
Barra Barra - Blackhawk Down Soundtrack
Jim & Geoffrey's Visit to the Highlands - Christine Fraser Ramsey
J'attends - Blackhawk Down Soundtrack
Mogadishu Blues - Blackhawk Down Soundtrack
The Cuckold Comes out of the Amery - Master and Commander Soundtrack
Moody Interlude
Synchrotone - Blackhawk Down Soundtrack
Tribal War - Blackhawk Down Soundtrack
Am I Born to Die - Cold Mountain Soundtrack
Strathgarry Set - Pete Clark
Christmas Time Will Soon Be Over - Cold Mountain Soundtrack
The Doldrums - Master and Commander Soundtrack
Great High Mountain - Cold Mountain Soundtrack
Man of Constant Sorrow - Oh Brother, Where Art Thou Soundtrack
Firefly Pilot End Credits
Far Side of the World - Master and Commander Soundtrack
Theme from Deadwood - HBO's Deadwood Soundtrack
You Will Be My Ain True Love - Cold Mountain Soundtrack
The Battle - Master and Commander Soundtrack
The Cuckoo - Cold Mountain Soundtrack
Native Funeral - HBO's Deadwood Soundtrack
Accidentals of the West - David Goodrich
Ride Across the River - Dire Straits
The Courier - Last of the Mohicans Soundtrack
I Will Find You - Last of the Mohicans Soundtrack
New Old Joe - David Goodrich
Promentory - Last of the Mohicans Soundtrack
Going Home - Gods and Generals Soundtrack
Folk Medley - Master and Commander Soundtrack
The Phasmid - Master and Commander Soundtrack
Lonesome Valley - Oh Brother, Where Art Thou Soundtrack
Spooks on the Hill - Open Range Soundtrack
Wagon Wheel - Open Range Soundtrack
The Kiss - Last of the Mohicans Soundtrack
Man of Constant Sorrow (instrumental) - Oh Brother, Where Art Thou Soundtrack