For when you're missing GrowlMail:
sudo aptitude install libnotify-bin
- Install the Mailbox Alert extension for Thunderbird
- Right-click on a mailbox in Thunderbird and select Mailbox Alert. Set it to run this command:
/usr/bin/notify-send --icon /usr/share/pixmaps/mozilla-thunderbird.xpm %sender %subject
Update: when I posted this, there were some problems with sending alerts from Mailbox Alert using notify-send, but as of v0.6 these have all been fixed.