XML does not render inside iframe using the built-in Firefox renderer as of Firefox 17.0.1
We make a Content Management system that renders XML inside of iframe in a few different areas. As of Firefox 17, these iframes no longer render using the built-in Firefox XML renderer but instead as plain text.
Wšě wotmołwy (2)
I've also verified that the same iframe when rendered alone works fine with the XML renderer and that I'm properly sending text/xml in both cases.
A good place to ask advice about web development is at the MozillaZine "Web Development/Standards Evangelism" forum.
The helpers at that forum are more knowledgeable about web development issues.
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It is probably best if you have a web page with example code, so they can test this themselves.