"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"