The situation is as follows: a website which, after logging in with a username and a password, allows you to register new users and a Mozilla Firefox user with the option to remember passwords enabled.
The case: the user adds a new account or logs in with an account and then edits this account from another account.
The problem: When the user is adding the new account Firefox will fill the fields with the user and password of the current account. When the user is editing an existing account Firefox will fill the fields with the username and password from the account being edited —which is also in the password reminder— if the user is not editing the account with the intent to change its password will not notice that only the first of the two fields "Password" and "Repeat password" is filled, so when the user tries to save it will display "Passwords don't match" error.