MelasZepheos said:
I don't know much about people doing several shows at once but from what I've heard it's very rare outside of cameos, and often leads to all sorts of conflicts.
It is quite rare, I read an interview with Casey Biggs where he mentioned that while he was playing Damar on DS9 he was also in another TV series and this resulted in an extremely hectic schedule - half the week was spent on one coast and half the week was spent on the other. Of course Damar was only a supporting character, primary characters are usually on set nearly every day so two shows could only be juggled if their production periods didn't overlap at all.
This isn't beyond reason, and if Firefly had short seasons (which is common with many high end cable TV series) Fillion may even have some time to breath now and then, but it does seem unlikely.
I love Castle, hopefully if Firefly ever did return it would be after Castle had run it's course. Even good police procedurals can get repetitive after three or four few seasons. (And who wouldn't want to see Stana Katic or Molly C. Quinn with a recurring role on a new Firefly?)