This is a tracking bug for this Mozilla bug https://bugzilla.mozilla.org/show_bug.cgi?id=402597 

Opening comment:
The tooltip seen here http://www.mozilla.org/access/dhtml/new/tooltip has the
following debug for the Accessibility::Event object.  Note that the any_data
field, the target of the object:children-changed:add event, is None. 

source =  [paragraph | ]
type = object:children-changed:add:system
any_data = None
detail1 = 2
detail2 = 0 

This is also the case for dojo tooltips (actually alerts) seen here
http://archive.dojotoolkit.org/nightly/dojotoolkit/dijit/tests/test_Tooltip.html
and the tooltips linked from
http://developer.mozilla.org/en/docs/Accessible_DHTML#Supported_roles .

A strange oddity is that object:children-changed:add events for the live region
examples were broken (pre-pyatspi migration for Orca) with respect to
user_data, but now they work.  Beyond event type, I am not sure how else the
two issues are related.