I think there are some misunderstandings about who makes TV series.
TV series are made by production companies. TV networks buy the series and air them.
Sometimes, the TV networks own the production companies. Such as Universal, which owns UPN and also owns the production company that makes "Enterprise." Universal also owns all of the rights to the "Star Trek" franchise, TV and movies.
But sometimes, you have a situation where one network owns a production company, but another network buys the series to air. Doesn't happen as much as it use to, now that Fox, UPN and the WB -- which all own production companies and make series -- started their own networks. Back in the days when NBC, CBS and ABC were the only networks, it happened frequently. However, even Fox, UPN and the WB today air shows made by production companies they don't own.
I don't believe Fox owns the production company that makes "Firefly." Joss owns that. He just sells the series to the high bidder, in this case Fox. Fox didn't order a full season of episodes to begin with, and has informed the production company that it won't order any behind its initial order. So the production company is not making any more episodes because it has no one to air them. If another network or cable outlet steps in and picks up the series, the production company can make more episodes. Fox won't get any money if someone else shows these episodes.