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).

Markdown that is super effective. Don't know why it is receiving negative reviews.

Does not work at all for me with local markdown files. The effect is that a popup comes up asking what to do with the file, and when selecting "Open in FF" it opens a new tab and brings up the same popup again. Using FF 52.6.0 on RHEL 7.4. I did not try markdown files from a HTTP server.

Encoding is not UTF-8

Nice add-on, woks fine for me. Maybe just missing the rendering of [TOC] tag.

For people who found FF keeps asking "what app should be used to open the markdown file", just check the solution (at least for Ubuntu) here: https://github.com/KeithLRobertson/markdown-viewer/issues/2#issuecomment-321940542

Utf8 - encoded Chinese documents display messy code, while GB - encoded Chinese documents display normal. Please fix it. Thank you for sharing this practical extension.
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The problem has been solved. An option to change the default encoding to UTF-8 in about:config: set the value of intl.charset.fallback.utf8_for_file to true. It is perfect now.

doesn't work at all

didnt work

it doesn't work at all.

Not working with this URL: https://gist.githubusercontent.com/anonymous/6033c7fcb0caf7de6f0eec4637e79756/raw/71a19f13314e0e4af8c0913dd7131465b5910488/ja_en.md
And, it chooses wrong encoding system for Japanese characters. You can also reproduce it with above markdown file.

It is a nice and quick add-on, but the only limitation is it looks like not well support non english charater unless encode under utf-8 with bom

中文无法正常显示.

This add-on just works, I can now view markdown files with no additional software.

It just displays .md files. Plain and simple – exactly what I searched for (there are MD editors with far more bells and whistles, if you want to, this one is totally minimalist).
Unfortunately, Firefox (at least under openSUSE Linux) has an issue with the MIME type. The solution linked here solves it: https://github.com/KeithLRobertson/markdown-viewer/issues/2
Oh, and btw, it only works (for me) with UTF-16, not UTF-8 as long as you don't manually add it a BOM, which is somewhat unusual … (can be fixed with " sed -i '1s/^/\xef\xbb\xbf/' foo ")