You can check if you have a user.js file in the Firefox profile folder that sets the <b>browser.sessionstore.resume_session_once</b> pref to true. *http://kb.mozillazine.org/browser.sessionstore.resume_session_once The user.js file will only be present if you or other software has created this file and normally won't be present. You can check its content with a plain text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started 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. You can delete the user.js file if you didn't create this file yourself. You can use this button to go to the currently used Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) *http://kb.mozillazine.org/Profile_folder_-_Firefox See also: *http://kb.mozillazine.org/Preferences_not_saved