Markdown Viewer Webext by Kulero, Cimbali

Markdown is a lightweight markup language which uses plain text to describe formatting information, such as # Heading, 1. numbered list, **bold**, etc. This add-on identifies markdown documents by the extension in the URL (.md, .markdown, etc.). When you navigate to a markdown document, if the content is plain text, this add-on formats it into HTML (with headings, bold text, etc.) as described in the document and displays it directly in your browser.

Features

• Renders markdown files on the web and local files (file:// URLs).
  ⚠ On Linux, due to Firefox limitations, the add-on will only work if Firefox considers markdown files as text. Fixes are easy and available on the addon’s page and on this SuperUser question.
• Respects website security, including Content-Security-Policy headers. When such a header forbids modifying the file (for example “raw” markdown files on github) then this add-on does not render the page, or indeed inject any code or do anything at all with that page.
• Pick your CSS rendering style, from default or github-lookalike.
• Pick your code coloring rendering style.
• Add your own custom CSS (in the addon preferences).
• Reloading a page maintains your scroll location − ideal for editing a page and seeing what changed.
• Supports header anchors (i.e. links).
• Supports dark and light CSS modes out of the box.
• Optional (and disableable) markdown features, such as checkboxes, emojis, footnotes, laTeX, and more.

Permissions
This add-on requires minimal permissions to do its job. More details on this github issue. Permissions are:

• Host permissions (or “access data on all websites”) is limited to markdown pages, to inject the rendering code. If you look at the manifest, this matches URLs such as *://*/*.markdown and *://*/*.md
• storage, to store preferences (including your personalized CSS) in your sync profile.