Firefly is actually quite a different case. The show was broadcasted out of order and canceled before its original run of the first season was finished, so that the last episodes were not aired.
So the fans very much wanted more stories with exactly those characters.
Serentiy now is the movie that finishes all the important plot threads of the show.
It's the weirdest story of an IP: Total failure on TV, box office bomb, but an absolute classic to many and in fact an almost complete and very much finished story.
Serenity brought the series all the closure it needed.
So neither in the speculations what will happen with the universe nor in what the one shot can achieve is this a comparable situation. Unless Adam solves all the Swan Song story threads in those 8 hours.