Vivek Jishtu ha desarrollado una extensión para Firefox que agrega el soporte Ajax a Yahoo! Mail.

Adds AJAX support to the current Yahoo! mail and makes it a little more
friendly. You can preview your messages without leaving your inbox. It
adds a small arrow next to your email list. Click on the arrow to
preview the message.

Currently the Instant Reply is being developed. I will update it as soon as possible or if I get enough people who want it.

Que se ve de la siguiente manera:

Via Ajaxian Blog.