Works great if you configure it properly on Linux using this guide: https://github.com/simov/markdown-viewer/blob/main/firefox.md. If you using Ubuntu probably you have the snap version. In that case in about:config (following the steps on the previous URL) on Firefox be sure to put the full path of the mime file avoid using ~ character. And after that if the addon is not rendering properly the md files, then go to the advanced options of the addon and give the necessary permissions on file access and site access. The addon is great but this process complicate things. However it is worth it!