You most likely have a malware infection. You should try the following steps to clean it up:

• Troubleshoot Firefox issues caused by malware
• Refresh Firefox - reset add-ons and settings

If this was the case why wouldn't a full scan with BOTH Microsoft Security Essentials AND McAfee Virus scan find it?

Most likely not. Always on scanners like McAfee aren't that great at removing infections that are already on your machine (and viruses are VERY good at hiding from them). Also, BTW, there is no need to have both. I'd just keep Microsoft Security essentials and remove Mcafee, having two anti-virus will just slow down your computer unnecessarily.

The tools I liked to you above aren't replacements for anti-virus, but they are tools designed to remove an existing malware infection, which is what it sounds like you have.

Do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked
• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware