App 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/Session_Restore

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

• App Tab Initializer: https://addons.mozilla.org/firefox/addon/app-tab-initializer/

Hi thanks for the reply. The problem seams to be elsewhere, and for the moment, seems to be fixed. Apparently installing themes interferes with the tabs too. I've reverted to standard Firefox and it seems to be working!

Not just this - depends how you quit - filed as bug report..

https://support.mozilla.com/en-US/questions/825628#answer-202340