Reboot again. If the problem is still there,

1) Look on your desktop. Do you see a folder called; Old Firefox? Look inside. Look for the folder with the latest creation date.

https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

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

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

2) 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.

Make sure you install Firefox properly and that you do not run Firefox from within the DMG (Disk Images) file.

Open the Firefox Disk Images file and drag the Firefox application to the Applications folder on your hard drive. You shouldn't double-click the Firefox application to run it from the Disk Images, but instead drag it out of the DMG folder.

• https://support.mozilla.org/en-US/kb/install-firefox-mac