See:

• https://support.mozilla.org/kb/how-run-firefox-when-your-profile-missing-or-inacc

This is usually caused by a problem with the profiles.ini file and the profile marked as Default=1 in this file is no longer present on the hard drive.

• Windows: %AppData%\Mozilla\Firefox\
  C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\
• Linux: ~/.mozilla/firefox/
• Mac: ~/Library/Application Support/Firefox/

You can use one of these to make Firefox create a new default profile or reuse an existing profile:

• Delete the profiles.ini file to force Firefox to create a new default profile
• Use the Profile Manager to create a new profile
  Use "Choose Folder" when you create a new profile to select the location of a lost profile and recover this profile

• http://kb.mozillazine.org/Profile_Manager
• https://support.mozilla.org/kb/Managing+profiles
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Can someone explain in a simple, step by step manner how to do this? I appreciate the help, but not sure of what to do exactly.

You would have to go this folder and delete the profiles.ini file. Windows will probably show this as a Profiles file as it hides the .ini file extension.

• Windows: %AppData%\Mozilla\Firefox\
  C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\

The "AppData" folder in Windows Vista and later Window 7+ versions is a hidden folder.

• http://kb.mozillazine.org/Show_hidden_files_and_folders

If you delete the profiles.ini file then Firefox will create a new profiles.ini file and a new default profile folder.

If there are other profiles present then you can try to recover some personal data like the bookmarks from that profile if necessary.

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile