It seems the Firefox does not allow the perspective and rotateY to come together for the transform property of the selector #thumbnail-slider .active .thumb. So you can make the following modification to the thumbnail-slider.css, and it will start working with the Firefox browser:
/*-- Start 3D --*/
#thumbnail-slider .thumb {
-webkit-transform: perspective(500px) translateX(1px) translateZ(-30px) rotateY(30deg);
transform: perspective(500px) translateX(1px) translateZ(-30px) rotateY(30deg);
}
#thumbnail-slider .active .thumb {
-webkit-transform: translateX(0px) translateZ(0px) rotateY(0deg);
transform: translateX(0px) translateZ(0px) rotateY(0deg);
}
#thumbnail-slider li.active ~ li .thumb {
-webkit-transform: perspective(500px) translateX(-1px) translateZ(-30px) rotateY(-30deg);
transform: perspective(500px) translateX(-1px) translateZ(-30px) rotateY(-30deg);
}
/*-- End 3D --*/