Private Browsing Mode data not properly cleared by New Identity
It looks like the Mozilla documentation we used in #9570 (moved) was incomplete. It turns out that much of the private browsing mode context is not cleared upon all windows being closed on New Identity if you are using the default configuration ('browser.privatebrowsing.autostart' set to true). It is also not cleared by our existing usage of the cookie and cache clearing APIs are also leaving data from Private Browsing Mode sessions in tact.
All of this data does get cleared if you are storing history (which is why I missed this so far :/)
It also appears that the Torbutton Cookie Protections window is also always empty if you are using the default configuration ('browser.privatebrowsing.autostart' set to true).
Good test pages for verifying this are: http://samy.pl/evercookie/ http://www.stevesouders.com/blog/2012/09/10/clearing-browser-data/