Of all the markdown extensions I like the way this one looks the most. I'm bummed that it totally doesn't work.

Just getting file:// URLs to open in firefox is a huge pain. Whenever I open markdown files, Firefox just wants to download them. See here to fix: https://superuser.com/questions/696361/how-to-get-the-markdown-viewer-addon-of-firefox-to-work-on-linux/1175837#1175837
Edit: This isn't the fault of the extension

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 the rest of my system. I then added "text/plain md markdown" in that file. The problem is due to the way firefox handles files and mimetypes.
Edit: Again, this isn't a problem with the extension but does make it tricky to set up.

With this extension, when I open a markdown page, about half the time I just get a blank white page and half the time I see the file. Whenever I enable markdown or change markdown settings, the page reloads and I almost always get back to the blank white page.

When I open the addon's settings page, there are some pretty bad graphical glitches, everything is squished together and the bottom border just keeps slowly inching farther and farther down my screen.