Error Code - S1L457 is not an error code from Firefox.

Can you grab a screenshot of that message?

If you get a pop-up message asking to update Firefox or plugins or scanning for malware then such a message is likely a scam and you should never respond to such an alert to avoid getting infected with malware.

• Only update Firefox via "Help > About" or by downloading and installing Firefox from the Mozilla server and never via a pop-up or link on a web page.
• Plugins should only be updated via the plugin itself or by visiting the home page of the plugin.

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

Here is the print screen you requested. I have NOT called this number or powered off the laptop. I'm currently running AVG (free version) and everything has checked out. Cor-el has recommended several different malware scan programs as well as an Anti-rootkit TDSSKiller. I am a total layman. Am I in over my head?

You can check in "Windows Control Panel > Programs" for recently installed programs to see if suspicious software shows up.

• Control Panel > Programs > Programs and Features > Uninstall or change a program
• Click the Installed column to sort by this heading

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

If using various malware scanners does not fix it or if you are blocked from installing those scanners then ask advice at a forum that specializes in malware removal.

• http://forums.majorgeeks.com/index.php - MajorGeeks Support Forums
• http://www.security-forums.com/index.php - WindowSecurity.com Security Forums
• http://www.bleepingcomputer.com/forums - Bleeping Computer Forums

First off, that message doesn't look legitimate to me. It looks like Malware, confirming what cor-el has posted about. IMO, the best course of action is visit the recommended Malware removal forums for help.