You'll need to override FirebaseListAdapter.parseDataSnapshot() to extract the right value from the DataSnapshot:

FirebaseListAdapter<String> listAdapter = new FirebaseListAdapter<String>
        (this, String.class, R.layout.menu_list_item, restaurantMenuRef ) {

    @Override
    protected String parseSnapshot(DataSnapshot snapshot) {
        return snapshot.child("category").getValue(String.class);
    }

    @Override
    protected void populateView(View v, String s, int position) {
        TextView menuName = (TextView)v.findViewById(R.id.menuName);
        menuName.setText(s);
    }
};