Mozilla - Making Opacity Of Images Change When Called

I have a script that makes opacity of images change when it's called, but the problem is, in IE work perfectly, but not in mozilla. Here's the script :
Code:
function submitkeytest(){
showbanners = true;
multy = false;
x = 0;
while (x<2){
document.getElementById('b'+x).style.filter = "alpha(opacity=100)";
document.getElementById('b'+x).style.MozOpacity = 100;
document.getElementById('b'+x).style.opacity = 100;
if(document.getElementById('b'+x).name.indexOf(document.getElementById('keyfield').value)<=-1){
document.getElementById('b'+x).style.filter = "alpha(opacity=20)";
document.getElementById('b'+x).style.MozOpacity = 20;
document.getElementById('b'+x).style.opacity = 20;
}
x++;
}}
What exactly did I do wrong?