Your site gist.githubusercontent.com replies with a CORS header Access-Control-Allow-Origin. Firefox therefore refuses to inject the add-ons scripts into the page, so your markdown never gets rewritten/beatified. If you save the file locally and view that in the browser, it should convert. But since your file is UTF-8 with no BOM, you will need to configure intl.charset.fallback.utf8_for_file to true. Please see the project README file for more details.