Download a fresh copy of Firefox ( http://www.mozilla.com/firefox/all.html ) and do a reinstall of Firefox on top of the current version.

If that still causes the same problem then do a clean reinstall.

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 before doing a scan.

• 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

I traced my problem to Norton Internet Security 2010 on Windows 7 Home x64 only. They worked fine until last definition update for x64 and Norton's kept removing xul.dll, stated that plugin-container.exe was a Trojan and removed it. Even removal and reinstall of Firefox does not help with Norton's removing the file as soon as I start Firefox. Norton marks parts of Firefox as malware and removes them. Vista Home 32 bit and XP seem to work fine. This is Firefox 3.6.15 that I have the problem with and Norton's Internet Security 2010 on Windows 7x 64.