# opening a new tab

self.driver = webdriver.Firefox()
self.driver.find_element_by_tag_name('body').send_keys(Keys.COMMAND + 't')

# this is the solution for MAC OSX. In other cases you can use the standard Keys.CONTROL + 't'

# opening a new webdriver

driver = webdriver.Firefox() #1st window
second_driver = webdriver.Firefox() #2nd windows