I really liked "Firefly," and I'm generally the kind of person who pines away forever when a beloved show is cancelled (see: Middleman, JLU, Invisible Man, Eerie Indiana, etc.).
That being said, I don't think I'd want them to bring back "Firefly." And it's all because of "Serenity."
Let's say they picked up where "Serenity" left off: I don't necessarily think the show would be bad, but it certainly wouldn't be the same after the changes "Serenity" made, and I don't think the differences would be for the better. The other option would be to make episodes set before "Serenity," which would always have the Sword of Serenocles hanging above it. Inevitably there'd be winks ahead to what we know is going to happen, and overall it would make the show rather depressing, I think.
Much as I hate to say it, I think we're better off with "Firefly" as a finite entity.
But damn it, bring back the Middleman!