Yahoo! Mail Watcher is a Firefox add-on that works like Hotmail Watcher: users will be announced by a sound and a notification when they will receive a email.
The Yahoo! Mail Watcher works only if you are logged in your Yahoo Mail account. This means that you don’t need to write your username and password in the addon.
With a single click you can see the received mail. The addon can be configured to show the notification in the status bar or in the action bar of the Firefox. The sound and the alert can be disabled.
You cand download it from here .