This is an issue with using iframes.
on the confirm page, change the form tag:
<form name="checkout_PP_LI_Hid" action="https://www.PayPal.com/cgi-bin/webscr" method="post">
to:
<form target="paypal" name="checkout_PP_LI_Hid" action="https://www.PayPal.com/cgi-bin/webscr" method="post">