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.

Rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default value.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences
• http://kb.mozillazine.org/browser.sessionstore.resume_session_once

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)

You can check for problems with the sessionstore.js and sessionstore.bak files in the Firefox profile folder that store session data.

Delete the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox profile folder.

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

Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost and you will have to recreate them (make a note or bookmark them if possible).

Try the following:

1. Go to about:config in Firefox
2. Locate the string browser.sessionstore.resume_from_crash
3. Change the string's value to false
4. Restart the browser

Please report back soon.

If the above answer doesn't work, then I think it is a malware or add-on issue.
Try the solutions below the lines.

Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• You can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

Please report back soon.

Well, I disabled UMC Shopping 1.0.2, which I think was triggering Outlook to go to a Skpe page or script. Thanks for your help.

Also, I don't understand "Go to about:config in Firefox", but I believe disabling UMC Shopping 1.0.2 has resolved the issue. I'll watch the issue a while longer before marking resolved.

Cor-el, I'm not sure that deleting prefs.js files, etc. helped. It wasn't until I tried safe mode and then disabled UMC Shopping 1.0.2 that the problem went away. Thanks.

What is the content of the user.js file if you inspect the file in a text editor like Notepad (right-click: Open with)?

user_pref("network.protocol-handler.warn-external.dnupdate", false);user_pref("network.protocol-handler.warn-external.dnupdate", false);user_pref("network.protocol-handler.warn-external.dnupdate", false);user_pref("network.protocol-handler.warn-external.dnupdate", false);user_pref("network.protocol-handler.warn-external.dnupdate", false);user_pref("browser.sessionstore.resume_session_once", true); user_pref("browser.sessionstore.resume_session_once", true);

The user.js file contains a browser.sessionstore.resume_session_once line (actually two) that causes Firefox to restore the previous session, just like I assumed.

• user_pref("browser.sessionstore.resume_session_once", true);

You can delete this user.js file.