硒代码不会终止

Question

以下代码不会终止。可能是什么原因呢？

from selenium import webdriver

from selenium.webdriver.common.keys import Keys

driver = webdriver.Firefox()

driver.get("http://www.quora.com/physics")

elem = driver.find_element_by_class_name("cancel")

#ele=elem[0]

print "done"