This could be the result of an add-on or malware on your PC. Let's test by disabling or bypassing certain customizations.

First, make a backup of your computer for safekeeping. To back up Firefox, see Backing up your information.

Next, try starting Firefox in Firefox Safe Mode. Be careful not to "reset" anything permanently if you didn't back up.

Does that fix it? If not, try creating a new (blank) profile: Managing profiles.

(To move key settings like bookmarks from your damaged profile to the new profile, see: Recovering important data from an old profile.)

If the problem persists, then I would suspect malware/spyware.

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