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
• http://kb.mozillazine.org/user.js_file

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)

I have checked those settings and it is set to false.

Is there a user.js file in the Firefox profile folder?

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)

Note that Windows hides some file extensions by default.
Among them are .html and .ini and .js and .txt, so you may see a name without file extension, i.e. user and not user.js.
You can see the real file type if you open the Properties via the right-click context menu

Another theory to pursue if the above doesn't help: Firefox may resume a previous session if it detects that it did not shut down correctly after the previous session (i.e., if it thinks it crashed).

I am having the same issue, and it appears my settings are set to false. However, I can't get to/find the Profile Directory.

Help > Troubleshooting Information, no option/drop down for Profile Directory.

Ignore the above, found it and solved the issue..

Thank you