Currently nsIAccessibleText is implemented via atk/nsAccessibleEditableText. The Wrap classes for various widgets delegate to that.
We need to remove the delegation and implement it directly on nsHTMLTextfieldAccessible and nsXULTextboxAccesible etc.
Ultimately we need to implement it on nsHTMLContainerAccessible for those times when Midas editing is active on the container.
Apparently this bug was fixed by bug 340829?