Probably Firefox is not shutting down properly.You will need to kill any Firefox or plugincontainer process with Task Manager.

Firefox should then start ok. Simply uninstalling & reinstalling Firefox rarely solves problems. If you do reinstall firefox or upgrade it, do NOT use any option to remove personal information, because that will destroy bookmarks, settings and passwords.

• See Firefox hangs or is not responding - How to fix_firefox-hangs-when-you-quit-it

I appreciate the helpful information and I was finally able to uninstall FireFox and reinstall it. Everything's working good now.  :-)

Glad it is fixed. Thanks for posting back.

For future reference note

• that if a re-install is required, it will often need to be a clean re-install
  Troubleshoot and diagnose Firefox problems_5-reinstall-firefox
  • that involves deleting program files
  • but retain the profile, do NOT use options to remove personal data
  • it is good practice to backup the profile first
    Back up and restore information in Firefox profiles
• Sometimes a Reset works, but that does cause some loss of information.
  Refresh Firefox - reset add-ons and settings
  (There are better but more complicated methods suitable for more advanced users )

Such issues can also be caused if the profile that is set as the default profile (Default=1) in the profiles.ini is missing.
A possible cause is a reset action that failed somehow to complete and update the profiles.ini file properly.

• http://kb.mozillazine.org/profiles.ini_file

Thank everyone for their help. The problem is solved and I'm a happy camper.