Testing:

Firefox and IE behavior on
javascript "onBeforeUnload" event

Click a link below to play:
Using event.returnValue as suggested by many many pages
First try (Mix-case body tag only)
Second try (Mix-case body tag @!#$% AND script tag)
Third try (Mix-case script tag only)

#4 try (Everything Lowercase, body tag)
#5 try (Everything Lowercase, script tag)

NOW, without event.returnValue (using return)
#6 try (Mix-case body tag only)
#7 try (Mix-case body tag @!#$% AND script tag)
#8 try (Mix-case script tag only)

#9 try (Everything Lowercase, body tag)
#10 try (Everything Lowercase, script tag)


This is #9 try:

<script type="text/javascript">
function ConfirmClose()
  {
    return "This will end your session.";
  }

</script>

</HEAD>
<body onbeforeunload="ConfirmClose()">