It may not be the cause of your immediate problem but I strongly suggest that you disable/remove your Popular Screensavers Plug-in - a part of the troublesome MindSpark family.

Google for information and removal advice. You could start with - http://www.herdprotect.com/npp5stub.dll-6d2d5521f8c15d587f05119c714f0a431d053be9.aspx

When does the window open? What were you doing at the time? What are your HomePage, and Newtab pages?

alan_r said

It may not be the cause of your immediate problem but I strongly suggest that you disable/remove your Popular Screensavers Plug-in - a part of the troublesome MindSpark family. Google for information and removal advice. You could start with - http://www.herdprotect.com/npp5stub.dll-6d2d5521f8c15d587f05119c714f0a431d053be9.aspx

I disabled the Popular Screensavers Plug-in - we'll see what happens.

I already did a full scan, also ran Malware bytes. Just ran the PC Cleaner Plus. No malware found. Let's see if anything changes over next day. thanks for your input.

FredMcD said

When does the window open? What were you doing at the time? What are your HomePage, and Newtab pages?

It happens for no reason. I wan't doing anything special.

Tabs are:

The Guardian SF Gate Inoreader

Are there any references to this URL on the about:config page?

• http://kb.mozillazine.org/about:config

Do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked
• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

All:

It does look as though the problem has sorted itself out. I was giving it a few days to be sure, but it hasn't happened since Thursday. I think it was alan_r's suggestion to "disable/remove your Popular Screensavers Plug-in" - that's all I actually changed in the system. Thanks all for your help.