hi AFox, it isn't possible to set up a validated firefox account without control over the actual mail address (since there is a confirmation mail sent out to that address that has to be acted upon to activate the account).

so if someone else has created an account under your mail address but wasn't able to activate it subsequently, you can simply go ahead and recreate the account with that mail address - that will generate another confirmation email that will be sent to you.

basically the same applies if the account under your mail address was really verified - in this case you can trigger a password reset procedure at https://accounts.firefox.com/reset_password which will bring the account under your control...

Unfortunately someone hacked my account and setup a fraudulent Mozilla account. They must have gone back into the account and reset the password after the initial setup. Therefore I want to close the Mozilla account

ok thanks for explaining, do you still have control over your mail address? (without that it wouldn't be possible closing the firefox account based on that address)

if yes: trigger a password reset at https://accounts.firefox.com/reset_password and follow through the steps setting a new password for the firefox account. afterwards log-in to the account with the new credentials at https://accounts.firefox.com/settings and close the account from that page.