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 currently used Firefox profile folder:

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

Windows hides some file extensions by default. Among them are .html and .ini and .js and .txt, so you may only see file name 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.

The user.js file is only present if you or other software has created this file and normally it wouldn't be there.

You can check its content with a plain text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started 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.

You can delete the user.js file if you didn't create this file yourself.

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

Brilliant! There was a user.js file that was created sometime this afternoon, not by me. I don't know how it got there. Many thanks for the solution.

You're welcome.

Was there only this single user_pref line in the file?

Maybe leave an empty user.js file and make it read-only to make it harder.

There was the following in the file: user_pref("yahoo.ytff.general.dontshowhpoffer", true); user_pref("browser.sessionstore.resume_session_once", true);

I don't use yahoo so I doubt if I need the first line.

The file had the time of creation this afternoon when I was away from my PC (probably in sleep mode) and I don't think Firefox was even running then.

I'll see if the file reappears in the coming days and if it does I'll create the empty user.js file as you suggest.

Did you ever install the Yahoo toolbar for Firefox as is sounds that this program created the file?

Maybe the file got modified and the new entry got added.

It is possible the Yahoo toolbar got installed accidentally when I installed something else in the past but I would have removed it immediately.

I see that I have a Yahoo Application State Plugin 1.0.0.7 installed set to 'Ask to Activate' but I do not know what this is and I'm sure I did not knowingly instal it. I presume I can remove this.

I think you are right that the user.js file got modified rather than created yesterday afternoon.