Sounds like Malware.

Install, update, and run these programs in this order. (Not all programs detect the same Malware.) They are all free for personal use, but some have limited functionality in the "free mode" - but those are features you really don't need to find and remove the problem that you have.
Note: If your Malware infection is bad enough and you are mis-directed to URL's other than what is posted, you may have to use a different PC to download these programs and use a USB stick to transfer them to the afflicted PC.

Malwarebytes' Anti-Malware - http://www.malwarebytes.org/mbam.php
SuperAntispyware - http://www.superantispyware.com/
AdAware - http://www.lavasoftusa.com/software/adaware/
Spybot Search & Destroy - http://www.safer-networking.org/en/index.html
Windows Defender: Home Page - http://www.microsoft.com/windows/prod.../default.mspx

Many thanks. The first one did the trick (Malwarebytes) Found 9 affected files actually so they are now gone and Firefox is up and running. RonSims

I found the same thing...after running two different malware programs something clicked and I double checked the address of my default homepage...turns out I'd entered www.googe.co.uk which was the problem. No malware by default but who knows what happens if you click through...

Please report misuse of the Firefox trademark in websites using the Mozilla's Violating Website Report page and in emails by forwarding them to Mozilla's abuse email.