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 often do start them over from a bookmarked folder so the are "clean" only one page per app-tab. Besides app-tabs can be accidentally closed like any other tab (use "Ctrl+Shift+T" to restore a tab, if you notice the loss immediately).

• https://support.mozilla.com/questions/848046

App (pinned) tabs and Tab Groups (Panorama) are stored as part of the session data [1] in the file sessionstore.js [2] in the Firefox profile folder [3].

Make sure that you do not use "Clear Recent History" 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

• [1] http://kb.mozillazine.org/Session_Restore
• [2] http://kb.mozillazine.org/sessionstore.js
• [3] http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox

It is also possible to use:

• Firefox/Tools > Options > General > Startup: "When Firefox Starts": "Show my windows and tabs from last time"

It will be solve when you reset preferences of your Firefox.

instruction of reset Firefox : https://support.mozilla.com/en-US/kb/Resetting%20preferences

This is a terrible explanation:

1. Where is "Tools"???????????? Firefox 5 does not have a "tools" option! 2. The URL's that you provided to not work? 3. We get it, your smart, we are not. Please just tell us how to fix the problem using a step-by-step scenario.

If you are not on Windows then instead of Tools > Options use:

• Linux: Edit > Preferences
• Mac: Firefox > Preferences
• Windows: Firefox > Options or Tools > Options

Press F10 if the menu bar is hidden on Windows and Linux

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