If you have unstyled XML, Internet Explorer applies its own XSLT transformation to it to generate a friendly tree interface. This has several distinct advantages and disadvantages that I'd rather not get into at the moment.
Mozilla on the other hand simply applies no stylesheet if you don't specify one. i.e. Everything is inline and unstyled.
With later 1.2 builds you can tell it to apply the tree transformation however, as it started coming with one that someone created.
I typically use Mozilla for all of my XML work, because it actually supports stuff.
IE does not support namespaces to the extent it knows to how render content in an XHTML namespace within a document, which makes it pretty much useless.
Mozilla on the other hand has no problem rendering a XUL document with inline SVG and MathML for example.
After that basic requirement, Moz has simple XLink support, RDF support, superior CSS1/2 (and parts of CSS3) support, and a good XSLT transformation engine (among others). (IE also has an excellent XSLT engine, but this is the only thing it really does have).
The list continues on and on - Mozilla is the only platform offering such extensive support for rendering XML markup.