Got it to work for Firefox Windows - eventually. Documentation on git not so good - watch the vimeo vid very carefully at start. It requires "Access your data for all websites". If you you go into the Addons > markdown viewer > manage > permissions: toggle "Access your data for all websites" it will NOT work. This setting will NOT stick. You will navigate, away come back, close firefox, whatevs and it will have reverted to OFF. HOWEVER, if you do as the video does at 00:30-00:40, you will receive the pop-up "Access your data for all websites" it will then work. NB. This is a brand-newey fresh install on windows. not a single other thing installed. It may well be a bug of firefox. I wasted a bit of time working this out. Given it's free, I prefer to give helpful feedback. Doc needs to be a bit more explicit - neither I nor AI picked it up. Thanks for the tool.

It doesn't support tables!

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.