I read the one star review. There are no instructions because none are needed. Associate ".md" files with Firefox in Windows. Click on an ".md" or ".markdown" file from Windows file Explorer, and it is rendered in Firefox! What else could a person want? I tried FF extension "Markdown Viewer Webext" and I was disappointed that it was a real pain in the ass to look at local files. I don't need a security warning or a special way to open local file.
Thanks!

No instructions for opening local files. Firefox by default wants to download any .md I try to open in it.