App (pinned) tabs and Tab Groups (Panorama) are stored as part of the session data [1] in the file sessionstore.js [2] in the Firefox profile folder [3]. Make sure that you do not use "Clear Recent History" to clear the "Browsing History" when Firefox is closed because that prevails and prevents Firefox from opening tabs from the previous session. * https://support.mozilla.com/kb/Clear+Recent+History * [1] http://kb.mozillazine.org/Session_Restore * [2] http://kb.mozillazine.org/sessionstore.js * [3] http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox It is also possible to use the "Show my windows and tabs from last time" setting, that is usually a more reliable way to restore session data. * [http://kb.mozillazine.org/Menu_differences Firefox/Tools > Options] > General > Startup: "When Firefox Starts": "Show my windows and tabs from last time" If you use cleanup software like CCleaner then make sure that Session is unchecked in the settings for the Firefox application.

Thank you for that. I actually found that the answer (which had been posted as a reply to somewhat different problem) is to stop using Cmd-Q and use Quit in the drop-down menu under Firefox