Absolute vertical centering technique with image. Why is Firefox zooming in different ways?

Question

Absolute vertical centering technique with image. Why is Firefox zooming in different ways?

Technique in practice (works great):

http://coding.smashingmagazine.com/2013/08/09/absolute-horizontal-vertical-centering-css (first example in the article)

Results vary:

Chrome / Safari "scales" 100% of the width / height of the image using this technique from the vertical center within the "mask" with a fixed height of 300 pixels and a width of 100%. Therefore, when you increase the width of a container element, the image is cropped the same above and below when it is scaled, but the left and right edges “stick” to the container (without horizontal cropping).

.container
{
overflow: hidden;
position: relative;
width: 100%;
min-height: 310px;
min-width: 462px;
}

img
{
position: absolute;
top: 0; right: 0; bottom: 0; left: 0;
display: block;
width: 100%;
min-height: 310px;
margin: auto;
}