What it does is...
Firefox won't remember any history for this session.
In a Private Browsing session, Firefox won't keep any browser history, search history, download history, web form history, cookies, or temporary internet files. However, files you download and bookmarks you make will be kept.
To stop Private Browsing, select Tools > Stop Private Browsing, or close Firefox.
While this computer won't have a record of your browsing history, your internet service provider or employer can still track the pages you visit.