Oh man! You made me want to watch my DVDs right now.
I immediately thought of "Trash." A beautiful schemer who has a past with the crew has a plan to steal an antique ray gun from a rich guy's house. I don't want to list any spoilers, but this is a great episode with plenty of potential for an adventure. Check it out!
I also thought of "Ariel." The ship's doctor hires the crew to break into a hospital so he can perform some tests on his crazy sister. As payment, Simon will direct the crew toward valuable medical supplies they could sell. I actually used this episode as the inspiration for part of the adventure included in the back of the EotE book when the PCs need to get into a hospital to find their Hutt contact.
As someone else mentioned, "The Train Job" is another episode that would be easy to convert into an adventure.
For me, Firefly represents exactly the tone I am shooting for in my play sessions.
I wish I could go back and watch the entire series for the first time. Enjoy!