It can't be checked off in settings.

"Open previous windows and tabs" is disabled in private browsing mode aka "Never Remember History".

I do not have the new window as private browsing. I still can't check it. Thank you for trying to help me. This is very frustrating.

Is browser.startup.page locked/grayed in about:config ?

You can open about:config via the location/address bar. On the warning page, click "Accept the Risk and Continue" to open about:config.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

• browser.startup.page
  [0:blank; 1:home page; 2:last visited page; 3:restore previous session]

See also:

• https://support.mozilla.org/en-US/kb/restore-previous-session

Thank you for trying to help me. I did change the # from 1 to 3, but it's still shaded and I can't check it off. I even restarted Firefox and still no luck.

Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile. Make sure you stay signed out of your sync account when testing.

While you are getting help with restoring your previous session automatically, could you check on manual restoration options:

(1) Are either of these are available (not hidden or grayed out):

• "3-bar" menu button > History > Restore Previous Session
• (menu bar) History > Restore Previous Session

(2) If they are missing or grayed, are there any previous session windows listed here:

• "3-bar" menu button > History > Recently Closed Windows
• "Library" toolbar button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows