Windows Firefox can display text files. Save a .md file to your hard drive. Open your file explorer and right-click the file. Set its properties to treat the file as a text file. (Windows)

If this is a text file then prefix the link with the view-source: protocol to open the file as a text file in a tab. You can usually copy the link to the clipboard via the right-click context menu (Copy Link Location).

• view-source:https://....

Where is the .md file coming from?

If it's from a site like Github that is conversant with the extension, the "Raw" link will serve the file as plain text (Content-Type: text/plain):

HTML: https://github.com/jscher2000/google-uk-search/blob/master/README.md Raw: https://raw.githubusercontent.com/jscher2000/google-uk-search/master/README.md

Some other sites may indicate the correct Content-Type but push the file as a forced download (Content-Disposition: attachment).

Yet others may send a non-matching Content-Type to bypass viewing in the browser.

If you'll allow a little self-promotion, I have an add-on for that. You can use it to associate text/plain with the .md file extension and to override forced downloading. (Those are separate features.)

https://addons.mozilla.org/firefox/addon/content-type-fixer/

After installation, it is dormant. Click the Zzzz button to wake it up and trigger the download. Then use the button's drop-down menu to view the log and you can associate the correct Content-Type with .md files.

Now... many sites also serve HTML pages with the .md extension, so you probably only want to turn on this new association when you run across a problem. You can turn off the extension completely the rest of the time, or use the Enable/Disable types list to manage .md.