You can remove the website from your saved logins using Firefox Lockwise. See Edit or delete a login with the Firefox Password Manager for more information.

If you are concerned about unauthorized access to your account, you may want to consider changing your password for your Firefox Account.

Thanks, but I'm wondering how they got my email address and how do I delete the account from that website.

Could be any number of things.

If you are concerned about the security of your computer, you could use a free malware scanning utility like Malwarebytes to see if there's anything funny going on there.

Did you check that this isn't related to one of your extensions that require an account for some eervice ?