It sounds like your account is already registered with the same email you’re trying to use for the app. Try selecting “Forgot Password” on the login screen it’ll send a reset link to your registered email. If you’re not sure which email was used, check any older Mozilla or Firefox emails (like verification or update notices) to confirm.

Once you reset the password, you should be able to sign in and download the app without issues. If it still doesn’t work, clearing your browser cache or trying from a different browser (like Chrome or Edge) often helps too.