Firefly (TV) - Why "Firefly"?

According to Joss Whedon, in his autobiography (which you can buy here):

[H]e warned the cast early on that there was a reason why he named the show Firefly, after the spaceship, and not after any central character. Adam Baldwin recalls Joss declaring, "Because I've had experience with that before and I don't want that. You're all expendable. If I choose, you can go at any time."

And, if you've seen "Serenity", you know he wasn't kidding.