Firefox will not remember multiple user names and passwords for same site, yet it used to do so. Any ideas?

I belong to several publishers website, and I often have to access more than one author's account, which requires that I use their user name (usually an email address) and their password. In the past, whenever I double clicked on the space for user name, I would see all of those user names appear that I use for that site. When I clicked on the one I wanted, the stored password would appear in the password window, and I could log in. Now, it only shows me the most recent user name that I have used. I don't understand how to make it do what it used to do. Any ideas?