Joss Whedon has pretty much ruled out another Serenity movie, much less a revival of the Firefly TV series. The audience just isn’t large enough for such a movie to be the kind of blockbuster the studios want and he’s moved on to other projects.
Randall Munroe at xkcd.com however has come up with an idea for a really killer Serenity sequel, featuring a member of the Firefly cast.