Hi,
for some reason autocomplete="off" is ignored into password HTML input by Firefox.
you can try to put autocomplete="off" on edit profile HTML form (
https://developer.mozilla.org/en-US/docs/Web/Security/Securing_your_site/Turning_off_form_autocompletion)
Edit profile form become from file /components/com_users/views/profile/tmpl/edit.php
This is a Joomla core file so we recommend you to override, so copy it into /templates/your-template-folder/html/com_users/profile/edit.php
I think Joomla need to add this, we will suggestion this modification at Joomla Dev Team.
Alternatives:
1. Javascript
2. System Plugin (this is most secure but very complex to develop)