You may have a corrupt profiles.ini file. Close Firefox. Open your file browser to this hidden location:

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

Locate the profiles.ini file. Then rename or delete it. Use this new profile as is. Is the problem still there?

I deleted all the files, but still getting the same error message.

Use these links to create a new profile. Use this new profile as is. Is the problem still there?

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/how-run-firefox-when-profile-missing-inaccessible

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

Okay I've tried all these things. I deleted basically every firefox related folder I could find, including the files mentioned in these articles. I also deleted the application and did a full restart of my mac, then tried to reinstall. Still got the profile missing error message. I also tried to activate the profile manager through terminal, but got the same message.

I called for more help.

Thanks for the help, hopefully something is figured out

I found a way to fix whatever permissions got corrupted in order to have firefox create a new profile. Basically, I deleted the firefox files in my library folder then followed the directions on this link and got it to work.

https://appletoolbox.com/2016/07/fix-corrupt-user-accounts-macos/#For_macOS_Sierra_High_Sierra

That was very good work. Well done.