Add this line of code to your PayPal button code to make it open in a new window when clicked:
target="_blank"
You would add this to the
<form target="_blank" action="<a href="https://www.paypal.com/cgi-bin/webscr" rel="nofollow" target="_blank">https://www.paypal.com/cgi-bin/webscr</a> method="post">