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.

Still does not support file:// type. sad,

This addon does more than I expected it would. Being able to set a great variety of themes (incuded a dark one) is a great plus.

This add-on is a subsitution of https://addons.mozilla.org/zh-CN/firefox/addon/markdown-viewer/?src=search

Unfortunately, that author didn't maintain the add-on, but It can open markdown files in a Frame, this is the most important reason for I still stay on Firefox 56 and not upgrade to Firefox Quantum.

Simeon Velichkov's markdown-viewer is compatible with Firefox Quantum, But it can not display markdown in a Frame, can you solve this problem?

Good, but it doesn't correctly interpret code blocks for python

so good.

Simply opened and rendered my file when needed for the local drive. Mission accomplished.

And now it works fine

Thank you so much for this addon :)

I did not success to configure the plugin (firefox Quantum), even when declaring the exact markdown complete URL as allowed. Firefox is still proposing for downloading the file, instead of displaying it.

Either the configuration is too complicated to perform (or too complicated for me) or the module does not work.

Problems with this plugin are endemic with Mozilla and Firefox and not the plugin.
Often, it does not work, because Mozilla Security.
This is because Mozilla think we trust them more than the plugin dev, which I personally never do.
Mozilla break userland all the time.
they break and nofix any bugs.
Mozilla send our data to Google without asking, & close no fix that always. WTF? I would like to view Markdown file _I_ write in a browser, often Mozilla say no. So I recommend any tool outside this browser. e.g. apt install retext which Mozilla cannot remote break every few months.