How to set path to non-standard installation of Firefox browser in Selenium Webdriver JS on Win7

I am currently instantiating webdriver as follows.

var webdriver = require('selenium-webdriver');
var driver = new webdriver.Builder().forBrowser('firefox').build();

I understand that webdriver will look for firefox in the standard Windows install location. How can I specify that webdriver use firefox in a non standard location, such as a portable app installed in a folder on the desktop for instance. I've seen some answers in python, but none for node.js. Thanks!