Go to Tools >> Options >> General >> in the option "When Firefox starts" change to "Show my Home Page"

More about:

• How to set the home page

Thanks. That is the setting. Any other thoughts?

Go to about:config page, search for browser.startup.page change to 1. Still in about:config, see the value of the browser.startup.homepage, is about:home ?

• about:config

You can check if you have an user.js file in the Firefox profile folder that sets the browser.sessionstore.resume_session_once pref to true.

• http://kb.mozillazine.org/browser.sessionstore.resume_session_once

Note that Windows hides some file extensions by default.
Among them are .html and .ini and .js and .txt, so you may only see the name "user" without file extension.
You can see the real file type (file extension) in the Properties of the file via the right-click context menu in Windows Explorer.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

You can reset browser.sessionstore.* prefs that are user set (bold) to the default value via the right-click context menu on the about:config page to make sure that Session Restore is enabled and working properly.

• http://kb.mozillazine.org/Session_Restore
• https://support.mozilla.org/kb/restore-previous-session

• http://kb.mozillazine.org/about:config

Those are the current settings.

Also, I just noticed that the Google page does come up briefly (about 1 second), than the last page from the previous session opens. I don't know if that's relevant.

Try start Firefox in Safe Mode:

• Firefox in Safe Mode

Try scann your PC, maybe the problem is a malware:

• Troubleshoot Firefox Issues Caused by Malware

Also:

• Reset Firefox