I have a configured nginx server returning the following headers when serving plain, non-UTF encoded text files. Content-Type: text/plain; charset=ISO-8859-1 Content-Disposition: inline When the text files contain ANSI escape sequences (Standard ECMA-48) Firefox will often ignore the server Content headers to use a guess determined by the file extension. But the guess is often wrong and forces the browser to download the file instead of displaying it in the tab. Cause: document Type: application/x-vnd.mozilla.guess-from-ext; charset=ISO-8859-1 Is this intended behavior and is there anything else I can do to encourage Firefox to display the text file in a browser tab? As this is not the same behavior with other browser engines such as Blink or Trident. 6 sample files, all refuse to display in Firefox except 'r5-WTF4.ans' https://retrotxt.com/e/