I'm getting unwanted tabs popping up in firefox. How can I prevent this?

Question

New tabs are appearing at random times in my Firefox window. I can leave firefox open, and come back to the computer and have new tabs that have appeared. I can be actively working in another tab, and have a new tab appear.
I think this started when a game from popcapgames.com was downloaded to my machine. This game was not ever run on the machine - as far as the downloader will admit.
I tried to access Spybot Search & Destroy from this machine, and I suspected that I was not getting the correct site, but a forgery instead. I used a different machine to get Spybot onto a memory stick and installed it onto my machine. I've run it several times, and it has removed considerable amounts of spyware from this machine.
But, I'm still getting unwanted Tabs popping up.
I've verified that the pop-up blocker is still set in firefox.

Answer 1

Do a malware check with a few malware scan programs.<br />
You need to use all programs because each detects different malware.<br />
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
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 <i>Popups_not_blocked</i> article.