I have three Yahoo Mail accounts, and the login fails for all three. Using Safari, all three logins are fine. This started yesterday after an ISP maintenance outage. My cookies are set for "Accept cookies from sites" and "Keep until they expire". I have ALWAYS used these settings, and Yahoo Mail has always logged in fine... until yesterday. All other Yahoo sites allow all of these same accounts to login (Yahoo.com, my.yahoo.com, answers.yahoo.com, finance.yahoo.com, etc.). That means the issue is Firefox-specific and Yahoo Mail specific.