Check that you do not use "Clear history when Firefox closes" to clear the "Browsing History".

• Tools > Options > Privacy: History: [X] Clear history when Firefox closes > Settings
• https://support.mozilla.org/kb/Clear+Recent+History

(clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords)

history > Firefox will: remember history

so, it's not that

Did you check the Clear Recent History settings?

To see all the History and Cookie settings, select:

• Tools > Options > Privacy > History: Firefox will: "Use custom settings for history"
• [X] Clear history when Firefox closes > Settings

Is History > Restore Previous Session enabled?

"Clear history when Firefox closes" is not selected and "Restore Previous Session" is disabled

I have clumsy posted two same questions. Problem is solved here https://support.mozilla.org/en-US/questions/927023