downloadChooseFolder now sets the string path to the pref store item.

function downloadChooseFolder() {

  const nsIFilePicker = Components.interfaces.nsIFilePicker;

  const nsIFile = Components.interfaces.nsIFile;

  var fp = Components.classes["@mozilla.org/filepicker;1"].createInstance(nsIFilePicker);

  var refLocalFile = Components.classes["@mozilla.org/file/local;1"].createInstance(nsIFile );

  fp.init(window, null, nsIFilePicker.modeSave);

  const nsILocalFile = Components.interfaces.nsILocalFile;

  var customDirPref = document.getElementById("downloadDir");

  if (customDirPref.value)

  fp.displayDirectory = customDirPref.value;

  if (customDirPref.value) {

   var fileCustomDirFile= refLocalFile.QueryInterface(nsILocalFile);

   fileCustomDirFile.initWithPath(customDirPref.value);

   fp.displayDirectory = fileCustomDirFile;

  }

  fp.appendFilters(nsIFilePicker.filterAll);

  var returnFilePickerValue=fp.show();

  if (returnFilePickerValue == nsIFilePicker.returnOK) {

    var file = fp.file.QueryInterface(nsILocalFile);

    var filepath = fp.file.QueryInterface(nsIFile);

    var currentDirPref = document.getElementById("downloadDir");

    customDirPref.value = currentDirPref.value = file;

    customDirPref.value = currentDirPref.value = file.path;

  }

}