Recently (last two FF versions), when updating as a non-admin Mac OS 15 user, there has been an issue with the PrivilegedHelperTool - a tool that allows non-admin users to provide an admin password and update files in /Applications. The Helper Tool accepted the password but then closed. Starting Firefox will only bring the Helper Tool (admin login) back again, making it impossible to run Firefox under this user. The admin user was not affected. The solution was to reinstall Firefox. I had to remove my profile files in Library/Application Support and restart a few times until I could regain Firefox. I have seen issues with the Privileged Helper Tool with another app, sometimes even corrupting the update. I hope this helps other people to address this and that the issue is resolved in future versions.