Mozilla has released Firefox 18.0 Final with Per Window Private Browsing.
http://www.mozilla.org/en-US/firefox/new/
http://www.mozilla.org/en-US/firefox/20.0/releasenotes/
http://www.mozilla.org/en-US/firefox/new/
http://www.mozilla.org/en-US/firefox/20.0/releasenotes/