Adicionando um arquivo utilizando a webapi do firefox OS

addFile.js

var sdcard = navigator.getDeviceStorage("sdcard");
var file   = new Blob(["This is a text file."], {type: "text/plain"});

var request = sdcard.addNamed(file, "my-file.txt");

request.onsuccess = function () {
  var name = this.result;
  console.log('File "' + name + '" successfully wrote on the sdcard storage area');
}

// An error typically occur if a file with the same name already exist
request.onerror = function () {
  console.warn('Unable to write the file: ' + this.error);
}