Hi,

It sounds like a setting was changed on your new tab page. These changes are usually caused by third-party toolbars or add-ons. I understand that you would like to know how to change it back. Not a problem, please follow the steps below:

1. In the Location bar, type about:config and press Enter. The "This might void your warranty!" warning page may appear.
2. Click I'll be careful, I promise! to continue to the about:config page.
3. Type browser.newtab.url in the search box to find it on the list.
4. Double-click on browser.newtab.url and type in about:newtab in the window that pops up.

Restart Firefox and open a new tab to make sure the change was made. If there is any issues with plugins please also see: Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

Please post back with your results, we are happy to help.

Sounds that you have some malware on your computer.

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