Some of these sites have complicated ways to cirvumvent browsers' pop-up blocking ability and it is not always possible for Firefox's built-in pop-up blocker to detect them all.

You should install the NoScript add-on (everyone should have this installed anyway for security sake, in my opinion) and only allow the minimum amount of scripts that you need for the website to work in the way you want. Beware that some sites will be designed deliberately so you cannot use it without enabling the Javascript that causes the pop-ups but most will be stopped by using NoScript.

It may take a while to get to know how to use NoScript properly so I recommend read the guides at http://noscript.net

I hope that helps you.

Also 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