If you are using sync; Shut down sync from here.

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

[v57+] Type https://accounts.firefox.com/settings<enter> in the address bar.

Delete Account: Look for Delete Account at the bottom of https://accounts.firefox.com/settings

Use these links to locate and remove any profiles.

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. How many profiles are listed? How many should be there? Also, open the profile folder in your file explorer.

https://support.mozilla.org/en-US/kb/recover-user-data-missing-after-firefox-update

Firefox uses two locations for the Firefox profile folder. One location for your personal data (e.g. bookmarks) and another location for temporary files like the disk cache.

Location used for the main profile that keeps your personal data (Root Directory on about:profiles).

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

Location used for the disk cache and other temporary files (Local Directory on about:profiles).

• C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>

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