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 your System Details List shows that you have a user.js file in the profile folder to initialize some prefs on each start of Firefox.

The user.js file is only present if you or other software has created it, so normally it wouldn't be there. you can check its content with a plain text editor if you didn't create this file yourself.

The user.js file is read each time you start Firefox and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

• http://kb.mozillazine.org/Preferences_not_saved

Another possibility is that Firefox is crashing during shutdown. Under those circumstances, Firefox will automatically resume your last session.

One way to test whether that's what happening is to have Firefox present a list of open windows and tabs after a crash instead of loading the earlier windows and tabs automatically. Note: I'm suggesting this as a diagnostic to see whether this is the problem, and not suggesting that you have to live with it forever.

There are a couple of steps to do that:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste crash and pause while the list is filtered

(3) Double-click the browser.sessionstore.max_resumed_crashes preference and change the value from 1 to 0 then click OK. The preference should appear in bold, indicating that it has been customized.

Changing to a 0 didn't make any change

My PC knowledge is very limited and your suggestion goes way beyond my capabilities! Could I perhaps just delete and reload Firefox?

Hi sieb, reinstalling Firefox will repair any damaged program files, but usually the problem isn't the program files...

To track down the user.js file, try the steps in this article: How to fix preferences that won't save.

Also, this is a bit random, but can you check the shortcut you use to start Firefox?

right-click > Properties > Shortcut
if it's pinned to the Task Bar, right-click "Mozilla Firefox" on the short menu

In the Target field, there shouldn't be anything after this:

"C:\Program Files (x86)\Mozilla Firefox\firefox.exe"

Did you remove the user.js file that show in the System Details List?

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)

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

As I said, I am not well versed in these things so I decided to reset Firefox to default and I did the job! Thanks to all for the help.