Files with mime type text/markdown should not prompt to download, but display as plain text files. That's more helpful and more consistent with other browsers' behavior.
As a stretch goal, Fx could parse and format markdown text, possibly side-by-side with the source.
This has been discussed in [Bug 1319262] Mime type text/markdown is downloaded instead of displayed.
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This can help a lot, specially if it render GFM Markdown