I use Thunderbird to read this forum and have gotten jealous of the Markdown formatting that the website supports but that Thunderbird doesn't. So I made an extension to render Markdown-formatted messages in Thunderbird.
Unfortunately this doesn't allow sending messages with Markdown formatting from Thunderbird—that would probably belong in the Markdown-here extension but can't work yet because Thunderbird currently doesn't allow modifying the content-type header (bug filed by Vladimir). My extension is limited to reading Markdown-formatted messages.