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Markdown Viewer on Firefox

Table of Contents

Compatibility Matrix

Origin Type Headers Render Autoreload
file:/// local requires mime type fix on Linux ✔ ✖
http(s):// local requires content-type: text/plain ✔ ✔
http(s):// remote requires content-type: text/plain + non strict CSP ✔ ✖
http(s):// remote strict CSP (Content Security Policy) ✖ ✖

Access to local file:/// URLs on Linux

Unlike Chromium based browsers, files served with text/markdown content type on Firefox will prompt you to download or open the file with an external app. For that reason markdown files have to be served with the text/plain content type instead.

The following are some methods to enable local file:/// access on Linux:

Method 1

Create ~/.mime.types file with the following content:

type=text/plain exts=md,mkd,mkdn,mdwn,mdown,markdown, desc="Markdown document"

Restart Firefox

Method 2

In case Firefox was installed as a snap Method 1 won't work.

Create /home/me/snap/firefox/common/mime.types file with the following content (replace me with your user):

type=text/plain exts=md,mkd,mkdn,mdwn,mdown,markdown, desc="Markdown document"

Go to about:config in Firefox and search for helpers.private_mime_types_file

Replace the default path there with /home/me/snap/firefox/common/mime.types (replace me with your user)

Restart Firefox

Method 3

Create ~/.local/share/mime/packages/text-markdown.xml file with the following content:

<?xml version="1.0"?>
<mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
  <mime-type type="text/plain">
    <glob pattern="*.md"/>
    <glob pattern="*.mkd"/>
    <glob pattern="*.mkdn"/>
    <glob pattern="*.mdwn"/>
    <glob pattern="*.mdown"/>
    <glob pattern="*.markdown"/>
  </mime-type>
</mime-info>

Execute:

update-mime-database ~/.local/share/mime

Autoreload on localhost

The autoreload feature is available only for content served on localhost and it won't work on file:/// URLs due to CSP (Content Security Policy) limitations.

Any file server can be used locally as long as it serves the markdown files with text/plain content type.

Here is an example file server using Node.js (replace me with your user):

var express = require('express')
var serveIndex = require('serve-index')
var serveStatic = require('serve-static')

express()
  // serve markdown files with content-type: text/plain in Firefox
  .use((req, res, next) => {
    // by default serve-static serves markdown content with content-type: text/markdown
    if (
      /Firefox/.test(req.headers['user-agent']) &&
      // the default Path Matching RegExp in Markdown Viewer
      /\.(?:markdown|mdown|mkdn|md|mkd|mdwn|mdtxt|mdtext|text)(?:#.*|\?.*)?$/.test(req.url)
    ) {
      res.setHeader('content-type', 'text/plain; charset=utf-8')
    }
    next()
  })
  .use(serveStatic('/home/me'))
  .use('/', serveIndex('/home/me', {'icons': true, view: 'details'}))
  .listen(8000)

Go to the Advanced Options page for the extension and add the http://localhost origin (note that port is omitted).

Run the above JavaScript file using Node.js and navigate to http://localhost:8000 in Firefox.

You can use any other host name configured in your hosts file that resolves to localhost:

127.0.0.1    ssd

Then you only need to add that origin http://ssd in the Advanced Options page as well.

The above script can be run on system startup using SystemD or any other service manager.

Remote origins with strict CSP

Remote origins that serve markdown files with text/plain content type and do not enforce strict CSP (Content Security Policy) can be enabled using the Advanced Options page.

For example, content hosted on gitlab.com and bitbucket.org can be enabled and subsequently it will be rendered:

Remote origins with strict CSP however, such as content-security-policy: default-src 'none'; style-src 'unsafe-inline'; sandbox will be blocked by Firefox.

For example, content hosted on GitHub's raw.githubusercontent.com cannot be rendered even if that origin was enabled in the Advanced Options page: