Example addon showing how to call the filepicker on Firefox for Android

filepicker.js

const Cc = Components.classes;
const Ci = Components.interfaces;
const Cu = Components.utils;

Cu.import("resource://gre/modules/Services.jsm");
function log(str) { Services.console.logStringMessage(str); }

function showToast(aWindow, message) { aWindow.NativeWindow.toast.show(message, "short"); }

var g;

function show(window) {
  var filePicker = Cc["@mozilla.org/filepicker;1"].createInstance(Ci.nsIFilePicker);
	filePicker.init(window.BrowserApp, null, 1);
	
	var rv =filePicker.show();
	
	if (rv == Ci.nsIFilePicker.returnOK || rv == Ci.nsIFilePicker.returnReplace) {
		var file = filePicker.fileURL.spec;
		showToast(window, file);
	}
}
	

function loadIntoWindow(window) {
	if (!window)
   		return;
	g=window.NativeWindow.menu.add("FilePicker", null, show(window));
}

function unloadFromWindow(window) {
  if (!window)
    return;
  window.NativeWindow.menu.remove(g);
}


/**
 * bootstrap.js API
 */
var windowListener = {
  onOpenWindow: function(aWindow) {
    // Wait for the window to finish loading
    let domWindow = aWindow.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowInternal || Ci.nsIDOMWindow);
    domWindow.addEventListener("load", function() {
      domWindow.removeEventListener("load", arguments.callee, false);
      loadIntoWindow(domWindow);
    }, false);
  },
  
  onCloseWindow: function(aWindow) {
  },
  
  onWindowTitleChange: function(aWindow, aTitle) {
  }
};

function startup(aData, aReason) {
  // Load into any existing windows
  let windows = Services.wm.getEnumerator("navigator:browser");
  while (windows.hasMoreElements()) {
    let domWindow = windows.getNext().QueryInterface(Ci.nsIDOMWindow);
    loadIntoWindow(domWindow);
  }

  // Load into any new windows
  Services.wm.addListener(windowListener);
}

function shutdown(aData, aReason) {
  // Stop listening for new windows
  Services.wm.removeListener(windowListener);

  // Unload from any existing windows
  let windows = Services.wm.getEnumerator("navigator:browser");
  while (windows.hasMoreElements()) {
    let domWindow = windows.getNext().QueryInterface(Ci.nsIDOMWindow);
    unloadFromWindow(domWindow);
  }
}

function install(aData, aReason) {
}

function uninstall(aData, aReason) {
}