Firefox 64 has removed the RSS/Atom feed preview and built-in Live Bookmarks feed reader functionality because of the low adoption rate. From now on, web feeds will be displayed or downloaded as regular XML files, depending on the server’s configuration.
For a better user experience, webmasters may want to replace direct feed links with subscription links of external services such as Feedly, or at least make sure that feeds are not downloaded. For your reference, the proper MIME types of RSS and Atom feeds are
Common feed readers may discover feeds with
<link rel="alternate"> so you should keep it.