In Options > General, I've always had 'When Firefox starts:' set to 'Show my windows and tabs from last time'. Recently however, on opening a new session, my apps have been lost - though not every time.
The Session Restore button is grayed out, and in the History menu, Restore Previous Session is grayed out.
Re: APPS NOT BEING RESTORED ON OPENING NEW SESSION
It is the same issue.
If you use "Clear Recent History" to close the Browsing History or session data stored in the file sessionstore.js is removed otherwise then no tabs are restored in the next session and that includes normal tabs as well as App tabs and Tab Groups.
If there is nothing to restore from the file sessionstore.js then "Restore Previous Session" will be grayed and the button won't show on the about:home Home page.