What do you have set here? see screenshot

Thank you for helping. Restore previous session is not checked.

In case you use "Clear history when Firefox closes" or otherwise clear history.

• do not clear the Browsing History

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

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

Hello cor-el,

I checked "restore previous session" Tabs remained. Sometimes it requires several open-close cycles for the tabs to vanish. I do not understand your above suggestions. Why should I clear history on closing? Thank you for your help.

You should NOT clear the browsing history when you close Firefox via "Clear history when Firefox closes" as that includes restoring pinned tabs and open tabs. You should keep pinned if you choose not to restore the previous session, but start with a blank tab or with the home page.

Yes, like this. see screenshot. All my pinned and regular tabs are open when I start Firefox.