The thunderbird dictionary page <https://addons.mozilla.org/thunderbird/dictionaries/?lang=en-US> currently says "These dictionaries work with the spell-checking feature in Firefox 2." It should say Thunderbird 2 (on this page). That page also need the "How to Install in Thunderbird:" info found on all thunderbird extension pages. In general all the "Install" links should read "Download" IMO (but that is a general issue, don't know if there's a bug for it.)
The Thunderbird-specific page is fixed with Remora. https://addons.mozilla.org/en-US/thunderbird/browse/type:3 The missing "How to install in Thunderbird" part is bug 371010, the change from install to a download links is bug 365550. And please file one bug per issue.