code: function installProfile(firefox, env) {
    var installed = promise.defer();
    child.exec(firefox + ' -silent', {env: env, timeout: 180 * 1000},
        function(err) {
          if (err) {
            installed.reject(new Error(
                'Failed to install Firefox profile: ' + err));
            return;
          }
          installed.fulfill();
        });
    return installed.promise;
  }
docs: Silently runs Firefox to install a profile directory (which is assumed to be
  defined in the given environment variables).
  @param {string} firefox Path to the Firefox executable.
  @param {!Object.<string, string>} env The environment variables to use.
  @return {!promise.Promise} A promise for when the profile has been installed.