well sometimes they do a secure login,add an additional layer of login. if they notice multiple login attempts or lot of weird activity ,maybe what u were upto check out for yet another avtar where they ask you for pasword alone or another one where they give a message saying do you find asnwering a few questions to ensure the login is right.
Yahoo! Mail has a special WYSIWYG editor for composing mails. Unfortunately Yahoo themselves have mentioned its supported only in IE, not in Firefox. But they claim to be working on one for Firefox as well. So no workarounds at the moment, other than waiting