if ( document.attachEvent )
{ // ie
   document.attachEvent('onreadystatechange', enableCategoriesIE);
} 
else if ( document.addEventListener )
{ // mozilla
   document.addEventListener('DOMContentLoaded', enableCategoriesFireFox, true); 
}

function  enableCategoriesIE()
{
    if (document.readyState == "complete")
    {
        document.getElementById(ddlCategories).disabled = "";
        
    }
}

function  enableCategoriesFireFox()
{
    document.getElementById(ddlCategories).disabled = "";     
}