I think it would add to the overall quality of the API if we could return more specific error codes rather than using NS_ERROR_FAILURE. One specific and common error is when API methods are called after the corresponding DOM node has been removed from the document (eg. bug 379872), for this error I suggest using NS_ERROR_DOM_NOT_FOUND_ERR, which prints as: Exception... "Node was not found" code: "8" nsresult: "0x80530008 (NS_ERROR_DOM_NOT_FOUND_ERR)" ... I think this is more informative than the generic NS_ERROR_FAILURE.

Looks reasonable for cross platform part since in gecko accessible objects and DOM nodes are close bound things. I don't know how to map it to HRESULT and I guess it doesn't makes a sense at all.

Are we planning to do anything with this, or is our defunct state enough to handle this case?

An accessible object may be dead for different reasons, it may be defunct even if its related DOM node is in DOM still. Also that affect our XPCOM API, which I wouldn't make much attention on it, as I'm not sure how long it will be kept alive. Suggesting to close this one as wontfix.