I am trying to center the background image both horizontally and vertically in the body tag and it works great in Safari, but not Firefox (I haven't tested IE yet). When you specify background-position: center center; (or even 50% 50% for that matter) Safari centers the image both horizontally and vertically in the browser window, and keeps it centered even as the window is resized. Firefox however only centers it horizontally. The vertical alignment in Firefox appears to align the center of the image with the top of the browser window, not with the center of the browser window.

How do I go about centering the background image on the body tag both horizontally and vertically in all browsers?