Working example of how to load the mobile version of a site using watir-webdriver 0.9.1, selenium-webdriver 2.53.0, webdriver-user-agent 7.4, and Firefox 46.0.1. (Version numbers important because Firefox.)

watir_firefox_mobile_example.rb

require 'watir-webdriver'
require 'webdriver-user-agent'

# Works as of:
# - watir-webdriver       0.9.1
# - webdriver-user-agent  7.4
# - selenium-webdriver    2.53.0
# - Firefox               46.0.1

# Creates new Firefox profile to bypass Firefox's chronic profile glitches.
profile = Selenium::WebDriver::Firefox::Profile.new
profile['browser.startup.homepage_override.mstone'] = 'ignore'
profile['startup.homepage_welcome_url.additional'] = 'about:blank'

driver = Webdriver::UserAgent.driver(:browser => :firefox, :profile => profile, :agent => :iphone, :orientation => :landscape)

browser = Watir::Browser.new driver
browser.goto 'tiffany.com'