I believe that problem results from clearing your cache or from the use of a program that cleans up your computer, such as CCleaner or WinUtilities if you are using a Windows machine. I had the same problem, as I always have several pinned tabs that would be gone as you state. I solved my problem with a work-around. I created a folder on the Bookmark Toolbar lableled Pinned Tabs and created in it shortcuts to all the URLs that I needed. When they disappear, I just click on the Folder and then on Open All in Tabs and then repin them all.

Thank you so much Bruce for your reply!! That seems a legit way to keep the tabs preserved :)

Actually, I didn't clean up my PC... I've just pinned a new tab and I've closed Firefox browser, then I open it again... Voila, my tabs r then all gone!!

In case you use "Clear history when Firefox closes".

• do not clear the Browsing History

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history

• Clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, passwords, and other website specific data.
• Clearing Cookies will remove all selected cookies including cookies with an allow exception that you want to keep.

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

Firefox uses the sessionstore.js file in the profile folder to store session data, but this file is only present when Firefox is closed. The sessionstore.js file is created from recovery.js when you close Firefox and is removed and copied to previous.js when you start Firefox to make is possible to restore the session at any time.

You will normally find these files in the sessionstore-backups folder:

previous.js (cleanBackup: copy of sessionstore.js from previous session that was loaded successfully)
recovery.js (latest version of the sessionstore written during runtime)
recovery.bak (previous version of the sessionstore written during runtime)
upgrade.js-<build_id> (backup created during an upgrade of Firefox)

You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.js to replace the current file (make a backup copy of the current sessionstore.js).

See also:

• http://www.ghacks.net/2014/06/27/mozilla-launches-improved-session-restore-firefox-33/

Thank you so much Cor-el for your helpful reply :)

Apart from the fact that all my pinned tabs disappeared and I cannot update Firefox I found that each and every time Firefox has updated it fouled SOMETHING up! Now I cannot even pin tabs after the latest failure. I am beginning to wonder if I should not go back to my previous Opera browser. After all why should I have to keep "repairing" Firefox each time it stuffs up?