Here we are:

 <!--

  if (dnn.dom.getCookie('myCookie') != null)
   document.location.href='Default.aspx?tabid=1'; //obviously you'll nee to change that to whatever your page is.
  else
   dnn.dom.setCookie('myCookie', 'true');

 //-->

Since I am not setting any expiration date to this cookie, as soon as browser is closed, the cookie expires. That way, the next time around, the user will again see the page.

NOTE: you'll need to make sure that dnn.js file is referenced on the page.