Hi MGB, For Mozilla account-related issues, please submit your question through this form. Under the "What do you need help with" field you'll want to select "Accounts & Login", and just summarize what you've already posted here in the body of the form.

The error message means that there is an account using that email address. There is an alternative site you could try. I will post links separately because they have to be moderated.

These are the links. Good luck.

https://accounts.firefox.com/signin https://accounts.firefox.com/settings

I was unable to add my secondary email address because of an error message stating that the secondary email account exists...

You may have created another Firefox Account (now called Mozilla Account) sometime ago and cannot use the same email address with another Mozilla Account.