Although I don't have a problem with them disappearing when I start Firefox, they always seemed like a potential problem over time. I use them but I generally do start them over from a bookmarked folder so the are "clean" only one page per app-tab.

• app-tabs https://support.mozilla.com/en-US/questions/837562
• app-tabs https://support.mozilla.com/en-US/questions/840004
• app-tabs improperly closing use History to restore - https://support.mozilla.com/questions/840583

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.

• https://support.mozilla.com/kb/Clear+Recent+History

I never do anything other then close the application. Next time I open it all the pinned tabs are gone. My normal home page tabs are still there. I have history set to clear each time it closes, could this cause the pinned tabs to be cleared also??

Yes, see my above post.

Ok then I just won't use it. Complete sillyness having a pinned tab be part of a browsing history session.

This doesn't happen every time I restart Firefox, however, it does happen quite often. Further, sometimes when I restart Firefox and lose my tabs, several full instances of Firefox open instead of just one. Also, I do not clear my browser history.