Okay first I'll just mention the tiny issues I have:
1) I keep getting ["Error saving — please copy your work to a safe place."] ... now this doesn't effect me since I only use this as a testing zone but it could become a problem later on if I do use it for actual documents.
2) This isn't really a big deal but I can barely hear the typewriter sound effects! D:

Okay now the reasons why I LOVE this add-on:
I was literally just looking for a markdown editor but as an extension, and for some reason I couldn't find one — until now!! Love it. The UI is great. The UX is intuitive. The cheat sheet is so helpful. Idk if this is universal to Firefox or the add-on itself but I'm so grateful clicking the icon opens in a new tab haha. .. I don't have any real problems with it since it satisfies my needs (again I just needed a wysiwyg testing zone), so all the other features are a plus for me especially the "chaos" ones :D

All I can say is 'Thank You'!

Very good ! It is the markdown online editor I want.

It's pretty awesome to be able to export Markdown to HTML!