I don't know what in the HTML tells Firefox that a field is a password to offer to remember, but I'm pretty sure it's possible to turn it off, as some sites fail to do this.
In 7 though, FF offers to remember it and this then causes interference with the later admin user account creation and logins, as far as I can tell.
I'm pretty sure older version of Drupal don't have FF trying to remember the database password.
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
Since there hasn't been movement in 11+ years think it's safe to close this out. Passwords being remembered seems like a browser feature not a Drupal one.
If anyone disagrees please reopen updating issue summary for D10 and up
Thanks!.