My first impression was that it was useless, because it states file:// URLs aren't supported. The docs on GitHub talk mainly about Chrome and the video has no sound, so I didn't find the workaround that somebody else posted in a comment. Here's the fix: https://superuser.com/questions/696361/how-to-get-the-markdown-viewer-addon-of-firefox-to-work-on-linux/1175837#1175837

The fact that this isn't clear in the instructions seems a pretty scathing oversight.

Anyhow, once I added the MIME type, it started working pretty well.