How Do I Stop A Form Submitting On Return

I have a form that submits on the user hitting the return key instead of the submit button. Searching revealed a javascript to stop it but it only works in IE not Firefox/Mozilla.

<SCRIPT LANGUAGE="JavaScript">

<!-- Begin
function onKeyPress () {
var keycode;
if (window.event) keycode = window.event.keyCode;
else if (e) keycode = e.which;
else return true;
if (keycode == 13) {
alert("Please Click ONCE on the Submit button to send this");
return false
}
return true
}
document.onkeypress = onKeyPress;
// End -->
</script>