It took me a while to even get it to do anything. I would appreciate proper instructions on how to use the extension.

This is not a problem with this extension, rather with Firefox. Whenever I open markdown files in firefox, it just wants to download them. I ended up having to change the mimetype text/plain to also include the extensions .md and .markdown. See here: https://superuser.com/questions/696361/how-to-get-the-markdown-viewer-addon-of-firefox-to-work-on-linux/1175837#1175837

The way the superuser article fixes the problem involves making all markdown files part of the text/plain mimetype but other apps on KDE (and possibly other 'DEs) use the text/markdown mimetype differently than text/plain. So I went into about:config and changed helpers.private_mime_types_file to a custom file (e.g. ~/.firefox-mime.types) to separate firefox's mimetypes from my system. I then added "text/plain md markdown" in that file. The problem is due to the way firefox handles files and mimetypes.

I am unable to change the code style in the markdown-viewer menu which is very frustrating, the markdown style does work.