Press the Login button, then press the Yahoo button and authenticate.
Despite a successful Yahoo login, I still get a "Login failed" message. Going by the below log, there appears to be a step where the Yahoo user id is null.
F22, firefox, release branch at 5a6f42d9dcee3b286ad88338e7df6675ee2393ff
This seems to be happening in fedora.zanata.org with FAS logins.