The user.js file is not normally included in a Mozilla profile dir, so if one tries
to use a a profile dir so that it can be deployed through a remote Firefox system the
open() triggers an exception about the user.js not existing:

from selenium.webdriver.common.desired_capabilities import DesiredCapabilities 
from selenium.webdriver.firefox.firefox_profile import FirefoxProfile
profile = FirefoxProfile(profile_directory="/home/myuserid/.mozilla")

selenium_browser = webdriver.Remote(desired_capabilities=DesiredCapabilities.FIREFOX,
                                    command_executor=http://anotherhost.com:4444/wd/hub',
                                    browser_profile=profile)