Position in fullscreen mode not working on Firefox

Question

I have implemented fullscreen on the site: http://nietkow.com (Just click on "Pantalla Completa / Pełny Ekran")

With these rules I have got the gallery centered on the screen in Chrome but not in Firefox:

.ad-image-wrapper:-webkit-full-screen {
    margin: 0 auto;
    width: 920px;
    background-color: black;
}

.ad-image-wrapper:-moz-full-screen {
    margin: 0 auto;
    width: 920px;
    background-color: black;
}

I have tried using different combinations and nothing seems to work. I can easily change the background color but not the position.

Is not clear to me what element is the parent in fullscreen mode and what rules apply to style both.