That's strange. Here is how you can reset what the tab does:

(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 newtab and pause while the list is filtered

(3) Double-click the browser.newtab.url preference and enter the desired value. Here are four possible choices:

(i) Page thumbnails (default)

about:newtab

(ii) Blank tab

about:blank

(iii) Built-in Firefox home page

about:home

(iv) Any other page

full URL to the page

Press Ctrl+t to open a new tab and verify that it worked. Fixed?

• If this does not take effect immediately, an extension may be overriding it. You may need to disable some extensions.

I suggest disabling ALL nonessential or unrecognized add-ons here:

orange Firefox button (or Tools menu) > Add-ons > Extensions category

Use one of the links above a disabled extension to restart Firefox.

• If this works until you exit and restart Firefox but then reverts back, you may have a user.js file entry changing it. This article has tips on that: How to fix preferences that won't save.

Thank you, worked straight away. Regards Anthony

Maybe be a malware check with some malware scanning programs on the Windows computer.
Please scan with all programs because each program detects different malware.

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
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender: Home Page:
  http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• 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

Hi Thanks for your suggestion.

I have Anti-spyware running and also a daily full scan together with Microsoft Security essentials again with a daily full scan. Neither of these identified a problem. An additional scan with Malwarebytes and Kapersky also did not pick up the problem or a solution. It was the first thing I went to.

The problem has been resolved.

Thanks for your helpful suggestion. Regards Anthony