I get this exact same error with thunderbird in 12.04:
(thunderbird:14459): GLib-GObject-WARNING **: cannot register existing type `DbusmenuMenuitem'
(thunderbird:14459): GLib-CRITICAL **: g_once_init_leave: assertion `result != 0' failed
(thunderbird:14459): GLib-GObject-CRITICAL **: g_param_spec_object: assertion `g_type_is_a (object_type, G_TYPE_OBJECT)' failed
(thunderbird:14459): GLib-GObject-CRITICAL **: g_object_class_install_property: assertion `G_IS_PARAM_SPEC (pspec)' failed
I get this exact same error with thunderbird in 12.04:
(thunderbird: 14459): GLib-GObject- WARNING **: cannot register existing type `DbusmenuMenuitem'
(thunderbird: 14459): GLib-CRITICAL **: g_once_init_leave: assertion `result != 0' failed
(thunderbird: 14459): GLib-GObject- CRITICAL **: g_param_ spec_object: assertion `g_type_is_a (object_type, G_TYPE_OBJECT)' failed
(thunderbird: 14459): GLib-GObject- CRITICAL **: g_object_ class_install_ property: assertion `G_IS_PARAM_SPEC (pspec)' failed