http://support.mozilla.com/en-US/kb/Private+Browsing The point of using Private Browsing is that it doesn't save any browsing history. Other than gaps in history there's really no way to know when Private Browsing was used, but any gap might be due to Firefox not being used. This extension disables Private Browsing, hides all the PB menu items and disables the keyboard controls, disables the clearing of browsing history, and the extension doesn't appear in the Add-ons except for the Firefox SafeMode. <br /> https://addons.mozilla.org/en-US/firefox/addon/disable-private-browsing/