Rated 4 out of 5 stars

For small documentation snippets it is a great tool, better than raw text files, because the relevant constructs are supported and look nice: images, simple tables, code, pre, bold, italic, etc.

It is not so great for bigger documents, as in-document-references are not supported. It autogenerates the ids for the headings ‒ #foo becomes <h1 id="foo">foo</h1> ‒ but it wont generate the href attribute on links ‒ [go to foo](#foo) becomes <a>go to foo</a>. Worse, using raw html links will also not work. From <a href="#foo">foo</a> it removes the href attribute! A workaround to prevent scroll scroll is the HeadingsMap Addon.

Other Issues:
It adds an unnecessary outermost div.container.
The css adds margins on left and right side. This can be fixed by editing the css in the xpi of this extension in the firefox profiles folder).
The title of the tab shows the contents of more than the first line of the md file (eg. '<title>======= loremipsum').

This review is for a previous version of the add-on (1.8.1-signed).