For a box office bust (unfortunately) Serenity is holding its own! For reference, here is Firefly's Amazon ranking history since it arrived on DVD. Its neat as you can see its sales ranking improve as Serenity approached and the impact Serenity had on the series.
Bottom line, TV shows don't do this. Canceled shows do not reappear on the silver screen. They don't beat out everything else (sales wise) out there two years after being canceled. It's great to see more and more people catching on. Now hopefully the executives at Universal (or Fox or SciFi Channel) start seeing that Firefly and Joss Whedon is something more than they thought.