The file:// URL issue is related to how Firefox handles markdown mime types. So unless Firefox do something about it I can do little, reminder, this is a Browser Extension for Firefox > 57, meaning that I don't have unrestricted access to the user's system.

About the blank page issue, that was caused by interference with other extensions and it was fixed on Chrome, I just have to publish the new version here.

About the settings page, you have to open it from the extension's popup in a separate window. The advanced options page is not optimized for the Firefox builtin extensions view.

EDIT: The 'Blank Page Issue' should be fixed now in v3.8