Initially planned for Firefox 3.0, this mode of navigation private was postponed due to time constraints. However, for the next couple of software, due at the end of the year, developers have now outlined to follow. Thus, during this session of surfing, all cookies will be stored and then deleted at the close of the application. In addition, the sites visited will not be recorded in the history of the browser. Accordingly, links clicked will not be marked with a color purple. When the user starts a closed session, tools backup passwords or automatic filling of forms will be disabled. Also note that the browser is closed, all downloads during this session will be emptied of the manager and all connections will be closed off.
You can find more details on the wiki developers.