Thanks for the response, but that didn’t work. After doing a little more research, I’ve found this thread in which Paul Irish says Firefox can’t transition background-image. Seems like something they should implement.
Edit: I have also tried using an element with fixed position set to cover the whole screen, then animating the opacity of that to achieve the fade effect, but what happens is it stays fixed until the page reaches the scrolling threshold where the fade happens, then during the fade it snaps to a different position. Then the body is in a different position as well, and after a moment it snaps to the correct place.