I just checked in a fix for #36025.
A sound will play when new mail arrives.
There is UI under "Edit | Preferences | Mail and Newsgroups" to turn it
off or on. It is on by default.
Eventually there will pref UI to allow you to choose any .wav file to play.
For you advanced users, you can edit your prefs.js file and do this now:
user_pref("mail.biff.play_sound",true);
user_pref("mail.biff.use_default_sound",false);
user_pref("mail.biff.sound_file","/home/seth/youvegotmail.wav");
Thanks to [EMAIL PROTECTED], who contributed the pref UI, and is
working on the full pref UI.
-Seth