The autocomplete="off" attribute was added to a few password fields but several browsers now ignore this for password fields due to suggestions of creating complex passwords for increased security or the inclusion of a password manager (see https://bugzilla.mozilla.org/show_bug.cgi?id=956906). There are some workarounds it seems that might be useful in the case of FireFox like http://stackoverflow.com/questions/17781077/autocomplete-off-is-not-working-on-firefox

For the rest of the browser fold it might still be wise to also add this attribute to the corresponding username fields in the ACP which would not have this attribute ignored but are still getting auto-filled with the username. The following fields would be the ones that would need the attribute added (there might be others):
Authentication -> LDAP user un (field ID: ldap_user)
Email settings -> SMTP username (field ID: smtp_username)
Jabber settings -> Jabber username (field ID: jab_username) -> Jabber password (field ID: jab_password)
Create forum (field ID: forum_image -> browser seems to treat this field as a username field)