I bought the series when it was a goldbox deal a couple months ago, the first episode is called Serenity. Is this DVD just that extended episode? The description on the back seems pretty similar to the beginning of the series. I don't want to buy something that I already have.
Serenity is the movie. It's the name of the spaceship, so that's why it shows up all the time. This takes place a few months after the end of the Firefly TV series. It's highly recommended. You will want to have it after you've finished the series.