It might not be too late.

Could you check whether either of these are available (not hidden or grayed out):

• "3-bar" menu button > History > Restore Previous Session
• (menu bar or Alt+S) History > Restore Previous Session

If they are missing or grayed, check either:

• "3-bar" menu button > History > Recently Closed Windows
• (menu bar or Alt+S) History > Recently Closed Windows
• Ctrl+Shift+N [reopen last closed window]

I successfully restored by going to the session backup folder and replacing with the before update backup file.

I wonder what made this happen and I am wondering if I should install an add on just to backup my tabs.

If it happens again, immediately Ctrl+Shift+N so you can close the private window first. Or get used to quitting Firefox using either:

• menu button > Exit
• (menu bar or Alt+F) File > Exit

Or look at an extension like https://addons.mozilla.org/firefox/addon/tab-session-manager/ that has an independent tab store. (I don't use it myself, so I don't know how automatic it is.)