The Download Now link will prompt a local download of the Firefox extension. To install the extension directly, open the file using your Firefox browser.
Wanna know more about a link or image you see in Thunderbird? Do you really really really wish there was a properties box with the info about objects? Well. Check out this extension. TB Properties allows you to see the image and link (and Web site) properties in Thunderbird. It gives you another Firefox function (assuming you have ThunderBrowse) in Thunderbird, today. Version 0.7 adds new languages.