Passwords are requested when first used, and rerequested if connection using what is saved fails. However if when you enter the new account wizard you click on the configure manually button at the bottom you will be shown a more detailed view in which you can set the user names to be used for incoming and outgoing. The wizard generally attempts to connect, which is why it asks for a password. Post setup, send a mail, if the incorrect password is saved you will be prompted to enter the correct one after the initial connection failure and you can select to save the password in the same dialog. Whilst the images are a version out of date, the information here covers the password manager. https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-tb