Firefox is just a web browser, it has no email features. If you are accessing email through Firefox it will be via a webmail service. Any features of the email will be part of the webmail service, you should contact them to see if they offer such a facility. There are add-ons such as [https://addons.mozilla.org/en-US/firefox/addon/4490/ WebMail Notifier] that check webmail services for new email, but I do not know of any that provide audio feedback.