If you start Firefox with "tabs from last time" you might be seeing them. However closing tabs on exit was not a specific feature, but closing history on exit was in the custom History settings: [about:preferences#privacy]

To reset the homepage/startup click on "Show my homepage" in the General settings.

Do you mean that tabs from the previous session are restored automatically on each start.

If the startup setting is OK (i.e. not "Show my windows and tabs from last time") then you can check this out;

You can check if you have a 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

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

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

shaw7429 said

I am using Firefox 39.0. In previous versions my tabs automatically closed upon exit. Is that option no longer available? I am unable to findanything in the help files.

I deleted the user file and the problem seems to be solved. Thanks for your help.