Most Thunderbird add-ons are available by using the Mozilla add-ons web site: https://addons.mozilla.org/en-US/firefox/. Next, download the Thunderbird Add-On. Click new Add-ons Manager – The newest Add-ons Manager is now able to find, download, and install Thunderbird Add-ons including Extensions, Themes, and Plug ins.
If you’re like me you are already saying just show me the bloody link, so here is the link for Markdown Thunderbird Add-on
Markdown Here
Totally straightforward to use:
- Create your email in your email client’s rich editor utilizing Github-flavoured Markdown.
- Right-click in the compose region and then click on “Markdown Toggle”.
- Your email is now pretty! (That is, it’s been rendered to Html code.)
- If you like the way it appears, just deliver it. If you want to modify or include anything, click on “Markdown Toggle” once again to get back again to your first Markdown.
You can take plain text like this: #Scheduling , with a flip of the toggle.
To this: 
Your emails will definitely get more attention and people might actually read them!
Resources and Links
If you’re new to Markdown, the Markdown Here Cheatsheet will make you a prostar.
For lots more info and advanced usage, see the Markdown Here project page.
So go have fun creating your own PRO-Looking MarkDown emails!