This is pretty good and does what I want. You can open a a markdown (.md) file on your machine in Firefox. There are multiple ways to style the page.

By default a small enlargable box is top left and you can click on this to get 3 options.

The first allows you to switch between the default and GitHub markdown style. (no utf-8 on one?).

The second has a decent list of styles that would be good but this didn't work for me.

The final option is a button to download the file as HTML. This works though the HTML is rather messy as it has all the code and styling for the dropdown box. You can remove the dropdown box though in the app’s settings though then of course you can’t click it to save as HTML. However you can delete the excess code from the file if you want to, though it's an extra step.

At least three other ways exist to to style pages. One is to write custom styles in the text box on the apps settings page. This is good for default styles.

Another way is to link to an external CSS file (put the link at the bottom of the page otherwise it won't work properly). You can also use an embedded stylesheet anywhere on the page (between two style tags).

All in all this is a great little add-on. Loses a mark from me because the dropdown styles didn't work for some reason and the generated HTML was messy.

Great extension.
Please, can you add markdown type mime "text/markdown" support (https://tools.ietf.org/html/rfc7763).
Thanks

Doesn't work. There's no button, and opening a local file on Linux still tries to download it.

Add-on works fine on Linux for local .md files using the mime type mod.
But where do you make the equivalent change for Firefox on OS-X?

The markdown viewer is amazing, very neatly done and robust. I even test it on a large .md file with no hiccups.
my only concern is a recent update recently, where a simple Markdown Viewer is requesting my permission to access personal data of all websites. I'm afraid this is a deal breaker for me. Why would a markup reader want to access my personal info of all my websites?
I'll gladly update my review and rating once that suspicious permission is tackled.
thank you

I use it to copy the webpage link into LaTeX with the hyperref package. I modify the default markup syntaxis with one appropriate for LaTeX (plus hyperref):

\href{${page.url}}{${page.title}}

Works perfectly on linux after adding mime type.
Don't forget to click on the toolbar icon to enable markdown rendering for local and web files

Not working. Tried to update mime database but firefox just won't render local and online .md files, even the README.md on github.

Exactly as advertised. I open .md or .markdown files in the browser and this "beautifies" them. Many thanks!

One suggestion I have is to have a "dark mode" that inverts the black on white or to have a "Custom CSS" box in the settings that would allow us to adjust this.

If this plugin is available on GitHub (haven't checked yet), I might consider making a pull request to implement the feature ^ mentioned above.

Edit: Fix typo

I cannot make this addon to display a markdown document which is located on my system.

Works fine for file opened locally - raw markdown is displayed initially and upon clicking icon it is rendered correctly.

If firefox does not correctly assume that your files are UTF-8 encoded, a possible solution (besides using a Byte Order Marker in your file, which is annoying for several reasons) is the following:
go to about:config and set the intl.charset.fallback.utf8_for_file variable to true. That worked for me.

This is related to the following bug report:
https://bugzilla.mozilla.org/show_bug.cgi?id=1071816

Note that the author notes that the extension only works "if the content is plain text". That means your browser needs to be rendering the file in a tab already, just without rendering the markdown elements. If it's downloading it instead, the suggestion from 7cac17 below, to add a MIME type for markdown files, worked for me (I'm also on Ubuntu).