May or may not be the issue, check Tools / Options click the 'Tabs' TAB

If 'Open new windows in a new tab instead' is checked, remove the check and click OK!

Thanks! Tried it. Problem still exists.

Check the Home page setting:

• How to set the home page - Firefox supports multiple home pages separated by '|' symbols

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

Ah-HAH! The Home page setting was all I needed to check. The culprit had inserted itself along with my default. Removing it seems to have solved the problem . . . I think.

Thank you very much!

You're welcome