User data is stored in the Profile folder which is not removed when Firefox is uninstalled. If you want a "fresh start" without any Firefox user data the two Profile folders (roaming and local file paths) need to be deleted.

https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Use these links to create a new profile.

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Type about:profiles<enter> in the address bar.

I deleted this folder:

C:\Users\<your Windows login username>\AppData\Roaming\Mozilla\Firefox\Profiles\

Now I'm receiving this error message (see attachment)

The Profile needs to be deleted before re-installing Firefox. When installing Firefox it looks for the presence of an existing Profile; when none is found a new Profile will be created.

You need to delete profiles.ini and installs.ini when present in the Firefox folder.

• C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\

• https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

I have already re-installed Firefox, but then received the error message listed in my reply above

I wound up resetting my PC (deleting all files and folders). It was a relatively new PC so this wasn't an issue. Thank you all for the replies, though.