App (pinned) tabs and Tab Groups (Panorama) are stored as part of the session data in the file sessionstore.js in the Firefox profile folder.

• http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox
• http://kb.mozillazine.org/sessionstore.js
• http://kb.mozillazine.org/Session_Restore

Make sure that you do not use Delete browsing, search and download history on Firefox to clear the "Browsing History" when Firefox is closed because that prevails and prevents Firefox from opening tabs from the previous session.

Thanks, but that's not the cause of the issue. I have never used "clear browsing history" or cleared it in any other way. And when I close Firefox, I never "close all tabs" if that option is displayed. The issue seems to be unrelated to anything I do, and it always happens if I install a Firefox update.

Question is still open.

It is possible that there is a problem with the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder.

Delete the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder.

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder
• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/sessionstore.js

If you see files sessionstore-##.js with a number in the left part of the name like sessionstore-1.js then delete those as well. Deleting sessionstore.js will cause App Tabs and Tab Groups to get lost, so you will have to create them again (make a note).

See also:

• http://kb.mozillazine.org/Session_Restore

The window with the Pin App tabs loaded must be the last window you close.

Firefox only remembers Pin App tabs from the final window you shut down.