I guess your best bet is to use a bookmarklet or install Greasemetal (which is Firefox' Greasemonkey for Chrome), combined with a script like XML Tree (old, but the source may still help). A more generic syntax highlighting script may help as well, but I doubt if you'll easily find one with code folding.
Note that whitespace might matter in XML. Not all XML viewers respect that; the screenshot created by the abovementioned XML Tree for the example XML does not respect it for the line Sample XML element containing a lot of text, enough to be put on a separate line.
(Unfortunately jsgui.com/xml-viewer is not responding while I am writing this.)