Simon is a doctor, but had to run away to save his sister. In Ariel, he dresses as a doctor to get River into a hospital to check her brain. This same doctor uniform is used as Dr. Horrible's costume in Dr. Horrible's Sing-Along Blog. But, here's Simon dressing as he used to, in Firefly.
More pictures after the break...
Screencaps from Ariel:
My Photobucket Album for Simon.
Serenifly Costumes site for Simon.