Mozilla New Mail Icon alias Biff is a Thunderbird extension that works on GNOME, KDE and IceWM, it docks Thunderbird into the system tray – – just like AllTray and displays an email icon (envelope) when new emails arrive in your Mozilla Thunderbird accounts.
Mozilla Thunderbird 2.0 already has a growl-like transparent notifier for incoming mails that works only when Thunderbird is open, and on your way. Mozilla New Mail Icon keeps Thunderbird out of your way, as it works even when Thunderbird is closed.
Installing the Mozilla New Mail Icon extension automatically disables Thunderbird’s in-built alert box feature. Uninstalling re-enables it again.
This is how your docked Thunderbird looks without an incoming email
This is how it looks when you have a new email.
Click on the docked-Thunderbird-icon to open or close/hide the Thunderbird email application.
To install this extension/add-on, download it, then go to Thunderbird, Tools -> Add-ons -> Install -> pick-up the downloaded extension and install.
If it took advantage of the growl-like notification feature introduced in Ubuntu 9.04, implemented for example in Gmail Notifier, it would have been almost perfect.
Mozilla New Mail Icon is released under the GNU/GPL license.