Created attachment 242898 [details]
Currently, mfenced operators are created as anonymous renderers and so are not searchable/selectable. See the attached testcase.
Actually, the normal <mo> operators currently contain anonymous text renderers and so are not searchable/selectable either (bug 125597). But even when bug 125597 is fixed, we would probably still have the problem with mfenced. See https://bugzilla.mozilla.org/show_bug.cgi?id=1108600 for the corresponding Gecko bug.