This could be due to a Firefox add-on or malware affecting your system.

1. To check if the problem is due to an add-on, start Firefox in Safe Mode. See these articles for instructions on Safe Mode: Diagnose Firefox issues using Troubleshoot Mode and Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.
2. If the first step does not help, scan your system for malware. This section lists some of the spyware removal tools: Spyware on Windows.

Do a malware check with some malware scanning programs.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of their databases before doing a scan.

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

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked