[Semi-OT] Markdown rendering extension for Thunderbird

[David Gileadi]

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](https://addons.thunderbird.net/en-US/thunderbird/addon/render-markdown-messages/) 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](https://addons.thunderbird.net/en-US/thunderbird/addon/markdown-here-revival/) extension but can't work yet because Thunderbird currently [doesn't allow modifying the content-type header](https://bugzilla.mozilla.org/show_bug.cgi?id=1717221) (bug filed by Vladimir). My extension is limited to reading Markdown-formatted messages.