It sounds like you have a third party program that has taken over your home page and/or the default new tab page. Fortunately, this can be remedied easily:

1. Click the menu button and choose Add-ons.
2. In the Add-ons Manager tab, select the Extensions panel.
3. Select the toolbar you wish to remove.
4. Click the Remove button.
5. Click "Restart now" if it pops up. Your tabs will be saved and restored after the restart.

After Firefox restarts, install the Search Reset Tool. This add-on will remove the rest of the traces of this program from your Firefox and then uninstalls itself.

For further information, please read Remove a toolbar that has taken over your Firefox search or home page.

Did this fix the problem? Please let us know!

thank you this seems to be working at the moment. Will let you know if it stops.

Glad that I could help.

If the issue doesn't come back, please remember to mark this thread as solved. This will help other users that have a similar issue to find their solution easier and more efficiently.

Thanks in advance. We hope that you will continue to enjoy Firefox.

after a week it seems to have changed itself back to using yahoo. This is really stressful to me

Do a malware check with several malware scanning programs on the Windows computer.

• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

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