This is a networking problem, not a Firefox problem, and it might be caused by something that affects every browser, such as a system-wide problem. If you had previously a virus that hijacked your browsers, then most likely this is something left behind by the antivirus, which removed the virus without setting network configurations back to normal. If you never ever had a virus, it can be a proxy problem, or a DNS problem.

Possible solution: you should review your network configurations, your HOSTS file (go here and select Fix It), or take it to a technician. I hope this helps.

Do a malware check with a few malware scan programs.
You need to use all programs because each detects different malware.
Make sure that you update each program to get the latest version of the database.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy
• http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page

See also "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked and What to do when searches take you to the wrong search website
If you can't fix it with the above listed scanners then you need to ask advise on one of the forums that specialize in malware removal mentioned in the Popups_not_blocked article.