Python Scraping JavaScript using Selenium and Beautiful Soup

I'm trying to scrape a JavaScript enables page using BS and Selenium. I have the following code so far. It still doesn't somehow detect the JavaScript (and returns a null value). In this case I'm trying to scrape the Facebook comments in the bottom. (Inspect element shows the class as postText)
Thanks for the help!

from selenium import webdriver  
from selenium.common.exceptions import NoSuchElementException  
from selenium.webdriver.common.keys import Keys  
import BeautifulSoup

browser = webdriver.Firefox()  
browser.get('http://techcrunch.com/2012/05/15/facebook-lightbox/')  
html_source = browser.page_source  
browser.quit()

soup = BeautifulSoup.BeautifulSoup(html_source)  
comments = soup("div", {"class":"postText"})  
print comments