Update: Since this was written, Firefox and Thunderbird have included Growl support by default on OS X and no longer require an extension to be installed.
Way back in November 2005, I wrote a guide to adding Growl notifications to Thunderbird, so that Thunderbird would issue a Growl notification when you received a new message. Though the method worked, it was very convoluted and not really for the faint of heart.
Thankfully, there’s now an official Growl extension for Firefox and Thunderbird. In Firefox, it’ll notify when a download completes, although Firefox 3 will have this feature built-in and so it won’t be necessary when that launches later this year. And in Thunderbird, it’ll notify when a new email message arrives. Setting up notifications in Thunderbird is now as simple as downloading this extension, installing it and restarting the program – no need to install any helper programs like growlNotify.
I’ll keep the old guide up for reference, but really everyone should use the official extension now as it’s much simpler.