This addon does more than I expected it would. Being able to set a great variety of themes (incuded a dark one) is a great plus.

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?

so good.

Simply opened and rendered my file when needed for the local drive. Mission accomplished.

And now it works fine

Thank you so much for this addon :)

Problems with this plugin are endemic with Mozilla and Firefox and not the plugin.
Often, it does not work, because Mozilla Security.
This is because Mozilla think we trust them more than the plugin dev, which I personally never do.
Mozilla break userland all the time.
they break and nofix any bugs.
Mozilla send our data to Google without asking, & close no fix that always. WTF? I would like to view Markdown file _I_ write in a browser, often Mozilla say no. So I recommend any tool outside this browser. e.g. apt install retext which Mozilla cannot remote break every few months.