They're frequently stored in the cookies. Although many such "cookie" passwords are saved, not all will be "Asked for" in the section you refer to. If you've set Firefox so it clears history when you log out, many of your cookies will removed and therefore, many of your automatic passwords will be removed also. Or if you manually cleaned out your cookies, it will do the same thing.
Hope this helps.
everything was fine yesterday, left pc on overnight. had to manually log into email this morning. check saved logins in FF options, everything is gone! what in the world happened? Could not recover from any restore point.