Do you mean client-side JS or server-side? If client-side, then I'm afraid you're out of luck - each of the browser brands features different XML support (some have no support at all), which is why the client-side examples that you come across are all for IE.
In order to maintain cross-browser compatibility, you need to perform the manipulation on the server so that the same result is delivered to all browsers. And even then, you can't just send a static stream of XML because of the varying support, so ultimately you have to transform the XML on the server into something more useful to the dumb browsers, such as (X)HTML.
This is why XML is best confined to server-side manipulation at the moment, possibly combined with optional support for client-side IE XML manipulation.
It is still not 100% clear whether you mean that you want to read cross-browser tutorials (which are therefore server-side), or ones specifically written for NS6/Moz (which I will refer to as plain old Mozilaa, since NS6 is just a repackaged and rebranded Mozilla) and which therefore disregard IE. However, I shall assume the latter, since my previous reply didn't seem to fit the bill.
I had originally believed that Mozilla was severely lacking in XML support, but further reading indicates that this is not quite true, although it's still not a patch on IE. A quick search on Google revealed these, which I thik may be of interest to you if you after Mozilla-specific XML code: