Why CSS3 perspective doesn't work in firefox

Question

I am trying to create a 3d flip card. But the perspective property does not work in firefox v21.

see code http://codepen.io/prantor19/pen/cgoKm

Can you explain why?

Answer 1

you forgot px after perspective!

like this:

-webkit-perspective:600px; -moz-perspective:600px; perspective:600px; 

Answer 2

 -webkit-perspective:600; -webkit-perspective:600px; perspective:600px; 

'perspective: 600px;' fine in chrome but "perspective: 600;" has a lie with this.