UPDATE: Download 0.9 from this page
I just uploaded the latest version of AJAX Yahoo! mail, its quite possible that this is the final/last version of this extension as the new Yahoo! Mail interface will be soon out of beta. But anyhow you would be able to download it from http://addons.mozilla.org as soon as it is approved. This version basically removes the problem faced by many where an empty box is shown instead of the actual mail.