Firefox stopped displaying XML files. How to fix?
According to this post, you have to add the <Doctype> tag to your XML document.
As a workaround, you can also right-click the page and select View Page Source.
This will actually be fixed in Firefox 74. See: https://bugzilla.mozilla.org/show_bug.cgi?id=1605657
There is no need to add a Doctype. XSD would be more appropriate for XML anyway.