Markdown Editor will show you a WYSIWYG preview of simple Markdown as you type. But it has a lot of limitations:

- Doesn't handle simple markdown like number lists (even though it handles tables pretty well).
- Does a poor job of rendering nested/indented lists.
- Its own default "Welcome to Markdown Editor" document has "An indented bullet" that doesn't indent!
- You can't simply open a local or remote .md file in Firefox and get Markdown Editor to preview it, you have to open the extension and copy and paste the markdown into it. (To be fair, last time I looked into it a decade ago, Firefox made it really hard for an extension to handle rendering a MIME type within Firefox.)
- It wants to manage its own set of markdown documents... somewhere, and then you can export these (only to your Downloads folder). I already have .md files all over my file system, I don't want to manage another set within a Firefox extension.

I had high hopes, but for many users it would be less hassle to write .md files as GitHub gists.