Rated 5 out of 5 stars
This is not a plugin - THIS is a serious reason to use thunderbird. Since this plugin exists for thunderbird, it makes thunderbird a top mail client for us keyboardists.
I could probably switch mail client, but I would never want to live without Nostalgy.
Thunderbird is simply a platform requirement for this mail client UI... ;-)
In short: Essential plugin for thunderbird if you are keyboard centric or has lots of mail.