You can check if you have an user.js file in the Firefox profile folder that sets the browser.sessionstore.resume_session_once pref to true. http://kb.mozillazine.org/browser.sessionstore.resume_session_once The user.js file is only present if you or other software has created it, so normally it wouldn't be there. you can check its content with a plain text editor if you didn't create this file yourself. The user.js file is read each time you start Firefox and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session. http://kb.mozillazine.org/Preferences_not_saved Or, if that didn't work make sure you don't have "Restore previous session" enabled. You can find info about that here: https://support.mozilla.org/en-US/kb/restore-previous-session#w_configuring-session-restore