This add-on is a subsitution of https://addons.mozilla.org/zh-CN/firefox/addon/markdown-viewer/?src=search

Unfortunately, that author didn't maintain the add-on, but It can open markdown files in a Frame, this is the most important reason for I still stay on Firefox 56 and not upgrade to Firefox Quantum.

Simeon Velichkov's markdown-viewer is compatible with Firefox Quantum, But it can not display markdown in a Frame, can you solve this problem?