Starting ~10 days ago, autofill in the address line (from from my contacts) doesn't work on my email (AT&T/Yahoo) on Firefox...works with Explorer - why?

AT&T Yahoo email account. Until about 10 days ago, when I created a new email, the address, CC and BCC lines would autofill contacts from my contact list when I began typing the name. Now, nothing happens and when I type the name, it turns red. The autofill function on my Yahoo account still works fine when using Explorer as browser, but I don't want to use Explorer! What gives? FYI, the email address line fills in fine on my iPad and iPhone...