Do you have Firefox set to start up with your home page or with your previous session windows and tabs? That is set on the preferences page, as described in this article: Startup, home page, tabs, and download settings.

If you have it set to home page, usually the History menu allows you to restore the previous session. Is that grayed out?

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

• do not clear the Browsing History

• Edit > Preferences > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data