See:

• https://support.mozilla.org/en-US/kb/restore-previous-session

Did you check "History -> Restore Previous Session" ? Are you closing Firefox via the menu or do you let Windows close Firefox?

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar, especially if you have multiple windows or Private Browsing windows open to prevent losing tabs in unnoticed windows.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

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

• do not clear the "Browsing history"

• Settings -> 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

• using "Clear history when Firefox closes" in Firefox 102+ honors exceptions and keeps cookies with an allow exception, previous versions removed all cookies
• clearing "Site settings" clears exceptions for cookies, images, pop-up windows, and software installation and exceptions for passwords and other website specific data

It sounds a bit like this other thread, which might make interesting reading (or commiseration?):

https://support.mozilla.org/questions/1434739

Thank you very much cor-el. I found my mistake with your help, but I couldn't come right away to thank you. Sorry.

Thank you for your useful suggestion, jscher2000.