A feature in Private Browsing called Tracking Protection blocks certain page elements. Most websites rely on many different "third-parties" - companies that are separate from the site you?re visiting - to provide analytics, social network buttons and display advertising. These third-parties sometimes include page elements that could record your browsing activity to create profiles about you across multiple sites and Private Browsing with Tracking Protection in Firefox Beta blocks some of these page elements.
Mozila says that Firefox Private Browsing doesn?t store information about your browsing session locally when you close the window. Private Browsing with Tracking Protection goes further by limiting data that third parties receive from you while you are in Private Browsing mode.
Mozilla is also experimenting with a new Control Center for Private Browsing in Firefox Beta that contains site security and privacy controls in a single place under the shield icon in your address bar.