Graphic designer hewtab is keeping the signal alive with her own custom Serenity shirt, featuring a Firefly in a jar. I see what you did there, hewtab.
The shirt is available for $20.99 from MySoti and comes in three colors (powder blue, navy blue, and black -- though I wish one came in brown). I think Firefly fans have been a largely untapped market for merchandise, so whenever I see something cool like this, I have to share it with my fellow Browncoats.
(Thanks to @ReelDistraction for the link!)