Components.utils.import('resource://gre/modules/utils.js');

var id = PlacesUtils.toolbarFolderId;

var writer = {
  value : '',
  write : function(aData, aLen) {
    this.value += aData;
  }
};

var result = PlacesUtils.getFolderContents(id, false, false);
PlacesUtils.serializeNodeAsJSONToOutputStream(
  result.root, writer, true, true, []);

var jsonString = writer.value;