I acquired the Opening Salvo book a long time ago, back well before I picked up Firefly. I was looking for a system that was different from the games like Pathfinder and Savage Worlds I was playing. I fell in love with the system even though I never managed to get a game going other than a one off. For a long time, I was hoping that MWP would come out with the Dragon Brigade RPG. Hopefully this is the next step.
This uses a similar system to Firefly, Leverage, and Smallville. It is a narrative system so it's not crunchy like Pathfinder and games like that. Instead, it's more like everyone is an actor. Because of the loose restrictions, anything is possible. I will be running a group shortly using these rules. I can tell you that it blends the old Cortex Classic System and FATE.
If you've picked up the Opening Salvo, a bit has changed. Unlike in the Opening Salvo, MWP has explained the Character Creation System very well. It doesn't take long to create a character and you can have your players in the action very quickly.
I don't remember Flourish Dice being in Opening Salvo, but they are a welcome addition to the system. If you are familiar with Firefly, these are the Banked Dice.
This book is enough information to get started. It has a little bit of background but could easily expand on it. I won't go into the adventure so I won't spoil it for someone, but I will say it's pretty good.