Perhaps your install of Firefox is hanging on exit, triggering the session restore.

You can disable session restore by changing a hidden preference.

   Type about:config into the location bar and press enter
   Accept the warning message that appears, you will be taken to a list of preferences
   In the filter box type resume to bring up a small number of preferences
   Double-click on the preference browser.sessionstore.resume_from_crash to change its value to false 

• solution courtesy TonyE

https://support.mozilla.com/en-US/questions/754840

Thank you for your response.

If there was a crash, it was not apparent, as I indicated in my note. How would I have known that it had crashed? In the past, when that happened, on restart, Firefox would show the "Well, this is embarrassing..." window, and let me choose whether I wanted to see the previous sessions, or not. Haven't seen that all these times the problem above has been happening.

I have now done what you suggest. I'll follow this for a few days. Let's see how it behaves from now on.

Thank you!

After a few days of running Firefox the way that was suggested, the problem doesn't manifest itself anymore. Thanks for your help!

Try to use File > Exit Firefox to close Firefox if you currently use the close X in the right corner of the title bar.

You can set the pref browser.sessionstore.max_resumed_crashes to 0 on the about:config page to get the about:sessionrestore page immediately with the first restart after a crash has occurred or the Task Manager was used to close Firefox.

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

That will allow you to deselect the tab(s) that you do not want to reopen, but will allow to reopen other tabs.
See:

• http://kb.mozillazine.org/Session_Restore#Restoring_a_session_after_a_crash