This is a big deal because not everyone knows about this feature. They might save their password to the browser without setting a primary password, which can cause a leak problem. So I recommend forcing everyone who wants to keep a password in the browser should set a primary password before using it.
Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.
@aloha I can totally agree that this feature is not visible enough! We can think of making some sort of message when user saves their login, to let them know there is such a thing as Primary Password and what it can do. But I don't think we should force it on anyone, the best protection is to lock OS user account when leaving device unattended.