getViewport - window.style.left - di Firefox

function getViewport() {
var viewPortWidth;
var viewPortHeight;

// the more standards compliant browsers (mozilla/netscape/opera/IE7) use window.innerWidth and window.innerHeight
if (typeof window.innerWidth != 'undefined') {
viewPortWidth = window.innerWidth,
viewPortHeight = window.innerHeight
}

// IE6 in standards compliant mode (i.e. with a valid doctype as the first line in the document)
else if (typeof document.documentElement != 'undefined'
&& typeof document.documentElement.clientWidth !=
'undefined' && document.documentElement.clientWidth != 0) {
viewPortWidth = document.documentElement.clientWidth,
viewPortHeight = document.documentElement.clientHeight
}

// older versions of IE
else {
viewPortWidth = document.getElementsByTagName('body')[0].clientWidth,
viewPortHeight = document.getElementsByTagName('body')[0].clientHeight
}
return [viewPortWidth, viewPortHeight];
}