Make sure to log out of Facebook before closing all Facebook tabs. Otherwise you would have to clear the Facebook cookies. Note that it might still be possible to show pages from the cache. You can consider to open the Facebook pages in a Private window or in private tabs. *Private Tab: https://addons.mozilla.org/firefox/addon/private-tab/ The Firefox menu button gets a purple background or you get a purple PB mask on the menu bar or tab bar (Linux) when you are in a PB tab and all Private Browsing mode tabs get a dashed underlining unless you are in permanent PB mode.