I recently used Firefox Nightly on my Android device, in a private tab, to login to gmail. After I closed the browser, both via the "quit" menu icon and via swiping the Firefox away in the Overview, I had expected the session information to be deleted and the next time I came back to gmail via a private tab, to be required to login again. However, this was not the case. Despite closing out the browser, something seems to have survived and the I was immediately logged back into the gmail session.

Is this some sort of expected behavior? Shouldn't closing out the browser delete all session information from a private tab? Is there something I missed that maybe I'm not actually "closing" the browser?