The nsExpatDriver:HandleError() method logs nsIScriptError messages with the sourceName URI pointing to the malformed document. This allows us to show such errors in the Web Console, if say the user opens up a page which is a malformed XML document. However, when the developer loads a malformed XML document with an XMLHttpRequest() we fail to show the message, since the sourceName URI doesn't point to any of the open tabs. There are other cases when this fails - eg. the use of document.load("malformed.xml")... or whatever we can think of. We need to figure out a solution for this problem. Perhaps bug 603706 would be the answer to this bug as well?
This is fixed by the patches from bug 606498.