You may have corrupt sessionstore file(s).

Type about:support in the address bar and press enter.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the sessionstore file(s). Then rename or delete them. Restart Firefox.

I am sorry, but this didn't solve the issue. I added a few screendumps of the settings. Another sollution?

Your More system details shows there is a User.JS file.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Do you mean that you can't reopen the tabs from the previous session?

• "When Firefox starts": "Show my windows and tabs from last time"

The last screenshot shows that you do not keep the browsing history (navigatie geschiedenis). This will prevent Firefox from restoring the previous session as this is considered history.

cor-el said

Do you mean that you can't reopen the tabs from the previous session?

• "When Firefox starts": "Show my windows and tabs from last time"

The last screenshot shows that you do not keep the browsing history (navigatie geschiedenis). This will prevent Firefox from restoring the previous session as this is considered history.

I corrected this setting, but the issue is still there

FredMcD said

Your More system details shows there is a User.JS file. https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

What software do you mean?

Protection programs sometimes cause problems by working too good.

Does Firefox create a sessionstore.js when you close Firefox?

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

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.

Backup the session files in the sessionstore-backups folder in the Firefox Profile Folder to make sure not to lose possible important session data. Do NOT close Firefox when Firefox is already running.

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/