var dom = document.getElementById ? 1 : 0;
var ie = document.all ? 1 : 0;
var mac = (navigator.userAgent.indexOf("Mac") > -1) ? 1 : 0;
var ieReal = (ie && (!mac) && ( typeof(window.offscreenBuffering) != 'undefined') ) ? 1 : 0;

function changeOpacity(obj, opacity)
{
  if(ieReal)
  {
    obj.style.filter = 'progid:DXImageTransform.Microsoft.Alpha(opacity=' + (opacity * 100) + ')';
  }else if(dom)
  {
    obj.style.MozOpacity = opacity;
    //obj.style.opacity = opacity;
  }
}