I played a lot of Star Frontiers back in the day, and the many published adventures for it could easily be adapted to Serenity. And, you're in luck, TSR allowed all the Star Frontiers stuff to be published for free on the Web! (registration required)

The Alpha Dawn adventures tend to not involve the PCs having actual spaceships/piloting skills, but the Knight Hawks ones do. The first three Alpha Dawn adventures involve being stranded on a frontier planet so aren't super "Serenity"-y but the rest, including Mission to Alcazzar and Mutiny on the Eleanor Moraes, are pretty directly portable.

I played a lot of the classic TSR space opera game Star Frontiers back in the day, and the many published adventures for it could easily be adapted to Serenity. And, you're in luck, TSR allowed all the Star Frontiers stuff to be published for free on the Web! (registration required)

The Alpha Dawn adventures tend to not involve the PCs having actual spaceships/piloting skills, but the Knight Hawks ones do. The first three Alpha Dawn adventures involve being stranded on a frontier planet so aren't super "Serenity"-y but the rest, including Mission to Alcazzar and Mutiny on the Eleanor Moraes, are pretty directly portable.