I can't get the bats or ghosts to work properly with Firefox either.
What I did to substitute was: rename the ghost1.gif to easter.gif
(I also reduced the image to half it's size)
Then open the 12.js script and change:
var no = 10;
10 represents the number of images on the screen, I changed it to 3.
also change:
setTimeout("snowIE_NS6()", 10);
10 represents the speed, I changed mine to 25 to slow them down a bit.
Re-upload the editted files, then in ACP choose Easter Basket as the Seasonal Effect.
It's a primative way to do it, but it serves it's purpose.