Growl and Thunderbird
Growl rocks. It’s one of my favorite add-on applications for OS X, but I’ve always wondered why it doesn’t have the ability to create custom notifications. Thankfully you can create custom notifications, it just turns out to be a pain.
The link below gives a not-quite detailed enough account of how to enable Growl notifications for new messages received in Thunderbird.
In order to make the setup work on my machine I had to enable the root account via the NetInfo Manager (found in Applications -> Utilities) and create a /local/bin structure inside /usr.
Be sure to follow the instructions closely. You generally don’t want to make mistakes as root.