For either latest Firefox or IE, I tried to inform the user that he/she has made changes on the current form, and if the he/she wants to save the changes.

So on the jsp, I coded:
<script>
window.onbeforeunload= function(){
if (checkAnyChange() == "true"){
displayCustomMessageModal();
return false;
}
};
</script>
The problem is that during the process, the browser displayed its own standard warning message such as displaying:"This page is asking you to confirm that you want to leave - data you have entered may not be saved." Leave | Stay

How can I suppress this message programmingly?

I do not want to manually disable the message, for I can not predict how the users will set up their browsers.

Thanks to help