According to the The Signal, a podcast dedicated to everything Firefly/Serenity, reports of the rumor that Joss Whedon cannot make any future Firefly episodes for television for X number of years is simply not true. They have a sound clip of Joss Whedon at WonderCon 2005 indicating that no such restrictions have been placed on him by anyone. You can here the comments on The Signal's Episode 12 podcast (Click link above) . So then, we can put that rumor to rest and hopefully look forward to some Firefly down the road, after the successful movie runs.
Les and Kari, hosts of The Signal, also revealed who it was that started that awlful rumor --- naughty, naughty Nathan --- you should be ashamed of yourself.