I was just testing to see if a CAP feed updated and realized that (at least using Firefox Developer 67.0b3) that the CAP XML does not display by default, but downloads. Is that something that is separated for speed purposes, like PDF or unexpected behavior out of an XML? Is an Add-On needed to enable that? Was that an intentional decision in the design?
I don’t think Firefox comes with a built in XML viewer anymore. So it will prompt you to download all XML files. If you want to view XML files in Firefox you thus need an extension for it.
You can possibly still view it in Firefox by adding view-source: in front of the URL (you won’t get any syntax highlighting most likely and extensions can’t do that for you afaik).
Firefox most definitely has an XML viewer. You can try it pretty easily: “data:text/xml,<root>something<root>”.
There is no more RSS viewer, so if a site sends RSS (marked with the RSS MIME type, not a plain XML type) Firefox will offer to download it. The removal of the RSS viewer was purposeful, yes.