I did not success to configure the plugin (firefox Quantum), even when declaring the exact markdown complete URL as allowed. Firefox is still proposing for downloading the file, instead of displaying it.

Either the configuration is too complicated to perform (or too complicated for me) or the module does not work.

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.