Firefox 5 has some animation support. I haven't had much time to play around with Firefox 5 yet, but from what I understand you would need to duplicate all the css with -webkit prefix and add a -moz prefix. You would also need to replace any translate3d, scale3d with their 2d equivalents (translate(x,y) or translateX() translateY() etc..) used with the -moz-transform.
It's probably a bug in that version of Safari/Webkit. My guess is that it is related to the "fill-mode" css property that is responsible for keeping the last animation keyframe styles active even after the animation finishes.
As for fixing it, you could try to apply the last keyframe styles to the elements directly (in the generated code) and see if that works for you.