All they need to do is say "we'll give you a nice juicy cut if we ever actually manage to make this darn thing return any money. If you want to be more involved, door is always open."
It's a no brainer that they have massive IP issues without consent to use various things, but given Fox canned the series and never funded a sequel to the movie, there certainly is scope for arguing fair use. Besides, not like the game devs could not (very properly) come up with a game that looks right, feels right, and is a ton of fun, but which does not actually impinge the IP. Fox does not own the concept of a motley bunch of cowboys in space, indeed the first time I ever saw Firefly the fist thing that came to my mind was "OMG it's a Traveller game!".
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