Firefox detected a potential security threat and did not continue to google
Open a new window in Firefox and type "about:config" without the quotes and hit enter Confirm that you want to continue In the search field, type in security.enterprise_roots.enabled and hit enter - you'll be left with one field If it's marked as "false" then double-clicking it should turn it to "true" Close the window, then go back to your Yahoo email login and try again. For me, this worked *perfectly*!