Joss Whedon recorded an introduction for the Firefly movie Serenity that was a thank you to the fans that had made it possible. It was shown during some of the early screenings, which is why he made reference to the movie not quite being done yet.
Note the line at the end about having done the impossible. A documentary on the whole Firefly/Serenity happenings took that as its title. Please see my separate posts on Firefly, Serenity, and Done the Impossible for more on them.