PythonExtensionFirefox.py

from selenium import webdriver
from selenium.webdriver.common.keys import Keys
from selenium.webdriver.common.desired_capabilities import DesiredCapabilities
import time
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC
from selenium.webdriver.common.by import By
from selenium.webdriver.chrome.options import Options

***** FirefoxProfile to add extensions *********

profile = webdriver.FirefoxProfile()
profile.add_extension('/Path to the extension.xpi')
profile.update_preferences()

desired_cap={ 'browser': 'Firefox', 'browser_version': '47.0', 'os': 'Windows', 'os_version': '7'}

******* Creating WebDriver instance ************

driver = webdriver.Remote(
    command_executor='http://<username>:<access_key>@hub.browserstack.com:80/wd/hub',
    desired_capabilities=desired_cap, browser_profile=profile
 )

************** Code Logic *******
driver.get('http://www.google.com')

driver.quit()