I'm giving it 4 stars because there is no dark mode.

Request for Dark Mode!

К сожалению, только просмотрщик, не редактор, и вспомогательный инструмент форматирования в произвольных полях текстового ввода.

Great add-on. To make it work under Ubuntu, follow this answer on superuser.com (less than one minute of additional work):

https://superuser.com/a/1175837

As stated, you will need to do a small tweak for it to work on Ubuntu, but it's only a matter of seconds.

Didn't get this to work. After installing it, for .md files, Firefox gives me two options: save the file, or open it with Xcode.

It just works. :-) Open a .md document and the document is rendered in Firefox. Don't want (or need) any more.

Сработало сразу и просто без доп. манипуляций с настройками.

does not work

Does not work if the .md extension is in upper case.

This plugin could have a longer description, but its functionality is pretty good. I was looking for a GitHub styled MD renderer, that
* supports math too
* works with my files on my drive directly (file:// and without utf BOM).

It isn't as fast as I expected, but still does a good job.