custom_firefox_path.py

from selenium import webdriver
from selenium.webdriver.firefox.firefox_binary import FirefoxBinary
driver = webdriver.Firefox(firefox_binary=FirefoxBinary('/Applications/Firefox 7.app/Contents/MacOS/firefox-bin'))
driver.get('http://input.mozilla.com/')
driver.find_element_by_id('show-custom-date').click()
driver.find_element_by_id('id_date_start').click() # should pop up a datepicker

default_firefox_path.py

from selenium import webdriver
driver = webdriver.Firefox()
driver.get('http://input.mozilla.com/')
driver.find_element_by_id('show-custom-date').click()
driver.find_element_by_id('id_date_start').click() # should pop up a datepicker