Firefox no longer places my password in the site. Do I need to reset something?

When I need to set a password for a site I've never visited before, Firefox asks if it should retain it for me. When I try to log on to a site I've been using for ages, it doesn't fill in the password for me. Has there been a change in version 18 or is it possibly something in the site I'm visiting? Most of these sites retain an automatic sign-in feature but they never retain the password so my user name shows up in the proper field, just not the password to get in.