How can you do this:

1. Click on the Menu History and select "Recent Tabs".
2. Select the tab title that you wish to restore.

If you are already in the new session:

1. Select the tab that you lost If you are in a new session: History > Restore Previous Session

If you would like to change the settings follow these instructions:

• Restore previous session - Configure when Firefox shows your most recent tabs and windows

In case you use "Удалять историю при закрытии Firefox" or otherwise clear history.

• do not clear the Browsing History

• Настройки > Приватность > Firefox will: "Будет использовать ваши настройки хранения истории": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history

• Clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, passwords, and other website specific data.