javascript cookie expiration in NS - WHY NOT?

How can I delete a temporary cookie in NS?

Can anyone tell me why the code below deletes 1 of the set cookies in IE, but not in NS?

<script language="javascript">
//set cookie
document.cookie= 'content=' + escape(location.pathname) + escape(location.search) + '; path=/';
document.cookie= 'url=' + escape(location.protocol) + escape(location.pathname) + '; path=/';
alert(document.cookie);

//delete cookie
var exp = new Date();
exp.setTime(exp.getTime() - 1);
document.cookie= 'content=' + escape(location.pathname) + escape(location.search) + '; path=/; expires=' + exp.toGMTString();

alert(document.cookie);
</script>

Thanks