You only keep the App tabs if you close (quit) Firefox with the window that has them still open.
App tabs may get lost if you close that window and still have the menu bar visible indicating that Firefox is still running (History > Recently Closed Windows).

Could you refer to a link in the help files explaining this ? My experience is that if I force quit FF , no app tab or group tab is saved Each time FF is closed and reopened, there is a risk that app tab is lost Therefore - app tab is a buggy feature

Does restoring tabs work or is that failing as well?

• Firefox > Preferences > General > Startup: When Firefox Starts: Show my windows and tabs from last time

You can check for problems with the sessionstore.js and sessionstore.bak files in the Firefox Profile Folder that store session data.

Delete the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox Profile Folder.

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

Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost, so you will have to create them again (make a note or bookmark them).

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

I have spent lots of time trying to save my App Tabs - It does not work. Useless. Period. I am using Zotero and Pocket extensions

Is there a sessionstore.js created with the correct time stamp if you close Firefox?

• http://kb.mozillazine.org/sessionstore.js

In case you are using "Clear history when Firefox closes":

• do not clear the "Browsing History"

• Tools > Options > Privacy: History: [X] Clear history when Firefox closes > Settings
• https://support.mozilla.org/kb/Clear+Recent+History

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder

Try this: http://www.youtube.com/watch?v=anUUg_8-Xr8