Totally Firefly/Serenity (or rather, ANY Joss show/movie) fan here!
I've found a lot of fanfic for it at
www.fireflyfans.net, but you have to really look hard for the good stuff. I've also found some gems at fanfiction.net . . . I especially recommend anything by Peptuck ("Forward" is especially good) or by mal4prez (the "Back Stories" series, which starts with "The Fish Job" is really good).
I've also written a few Firefly vignettes myself, and those can be found at either fireflyfans.net or at my profile on FF.net.