The Download Now link will prompt a local download of the Thunderbird extension. To install the extension directly, open Add-ons from the Tools menu. From the options button next to the add-on search field, select "Install Add-on From File..." and locate the downloaded add-on.
Muttator is a free browser add-on for Thunderbird, which makes it look and behave like the Vim text editor. It has similar key bindings, and you could call it a modal mail client, as key bindings differ according to which mode you are in.
