There aren't quite as many extensions for Mozilla Thunderbird as there are extensions for Mozilla Firebird but I just discovered one today that I really like. If you use Firebird and you haven't tried out Quick Reply then you're missing out. Qhick Reply gives you a small text area, directly below the message you're reading, where you can write and send a quick reply without having to open a compose window. I use the three vertical pane layout in Thunderbird and I think that QR fits the best in that view. You can find more information on QR at the Quick Reply home page. Great work, Erwin and David!
Do you use any Thunderbird extensions? Which ones? What features would you like to see implemented as extensions?