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.
From ingomueller.net:
ThunderBirthDay is an extension that displays birthdays from the Thunderbird address books as events in Lightning. Install Lightning, MoreColsForAddressBook and ThunderBirthDay, and then create a new local calendar choosing ThunderBirthDay as provider.
