Javascript get image height works in all browsers but firefox...please help?

So I am fairly new to Javascript (a lot of experience in PHP), but I have this basic script that checks a link that redirects to a URL to see what the image height is. If it is a certain height then it adds one to a variable, otherwise nothing. I would easily do this in PHP but the images are on other servers 开发者_运维技巧and not my own so it doesn't work.

Anyways, ehre is the script. Let me know if you have any tips. Works well and tested in Chrome, Safari, Opera, and IE.

<script language='JavaScript'>
window.onload = function() {

var nstar = 0, urls = [];
urls[0] = "http://optout.imiclk.com/cgi/nai_status.cgi?nocache=";
urls[1] = "http://www.adbrite.com/mb/nai_optout_check.php?nocache=";
urls[2] = "http://events.adchemy.com/visitor/auuid/nai-status?nocache=";

function getImgSize(imgSrc){
    var newImg = new Image();
    newImg.src = imgSrc;
    return{height:newImg.height, width:newImg.width}
}

for(i=0,length=urls.length;i<length;i++){
    if(getImgSize(urls[i]).height==43){nstar++;}
}
document.getElementById('tracknum').innerHTML = "<b>" + nstar + "</b>";

}               

</script>

Maybe the image isn't loaded yet? Try :

image.onload=function() {
    alert('W:'+image.width+', H:'+image.height)
}