Have you noticed any pattern to it?

Over the years users have reported problems with anti-keylogging programs that shift all characters to different characters, but yours sounds a bit different.

To reply I am composing my message in notepad and copying to the message box. The characters are so scrambled in Firefox that it is impossible to convey what is happening. I tried refreshing and got nowhere. I tried clearing the history cache, restarting in safe mode and rebooting. No joy!

Do you have multiple keyboard layouts installed in Windows?

If that is the case then you may have switched to a different layout. Windows remembers the keyboard layout per application.

Do a malware check with several malware scanning programs on the Windows computer.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure 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

I do not have multiple keyboard layouts. Keys are working now. There may be key control sequences that change the encoding that have not been documented well. There was malware of some kind present. I ran several programs with my Internet router turned off so that no outside communication could occur. Spybot found several inexplicable rootkit items that were removed. After more than six hours of scanning and several restarts things became stable and controllable. I ran a tuneup routine. Then enabled the router and updated all the anti-malware software and repeated the entire routine. Whatever specifically was causing the problem may never be known. The problem seems to be gone. The lesson learned is don't override blocks by the anti-malware software no matter how safe you may think a site is. Anyone can get hacked.