Firefox not displaying XML file as a tree

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I usually troubleshoot XML files by simply dragging them into a Firefox browser window. Unlike other text files, Firefox displays XML in a handy tree format, as show below.

Screen shot 2013-02-05 at 1.40.05 PM.jpg

Any syntax errors or typos in the XML result in a clear indication of where the problem is.

Screen shot 2013-02-05 at 1.41.24 PM.jpg

So I was surprised when an XML file refused to display as XML in Firefox. Instead, it showed up as the unhelpful run-together text below:

screenshot 2013-01-30 at 1-30-13 - 9.08.51 AM .jpg

It turned out that certain versions of Firebug will interfere with XML rendering. The bug was supposedly fixed, but appears to be still active in some cases.

Disabling Firebug will bring back the XML tree display.