Do you have anything under History > Recently Closed Tabs/Windows?

If you use "Clear history when Firefox closes":

• do not clear the "Browsing History" as that will prevent tabs from being reopened.

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

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords.

I have the same problem and have tried your solution - it did not work.

It matters not how many tabs are opened, upon restarting the last opened (active) tab is always replaced by the home page, in this case google.com

Did you check the startup setting?

• Tools > Options > General > Startup: When Firefox Starts:

Can you restore that session via History > Restore Previous Session?

• https://support.mozilla.org/kb/Session+Restore

Thanks - I checked your suggestion: "Did you check the startup setting?

   Tools > Options > General > Startup: When Firefox Starts:"

Mine is set to: "Show selected Session Manager session"

I changed it to: "Show my windows and tabs from last time"

Upon restarting this setting restored ALL tabs correctly. So it appears it is a "Session Manager" add-on conflict.

I will monitor over the next few days, enabling and disenabling the "Session Manager" add-on and ask again if problem reoccurs.

Thanks for your help.