Contact your online email provider about the login issue. No one user here on the forum can do help with accounts issues. And no one here knows what your email is yours and not yours either.

I am trying to set up a secondary account and keep getting an error message "This email was already confirmed by another user". How is this possible for 2 people to have the same email address? I use this email all the time so I know it is mine.

Its quite possible you may have created another account with your other email yourself at some point and forgot?

The message generally means you cannot use the same email for more than one account.

Check to make sure it isn't set as the secondary email on your existing account:

https://accounts.firefox.com/settings