A number of sites are using various means to get around Firefox's popup blocker. You can try extensions like adblock: https://addons.mozilla.org/en-US/firefox/addon/1865/ but sites are getting more and more savvy and may still get around it.

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