en_firefox.rb

Capybara.register_driver :en_browser do |app|
  require 'selenium-webdriver'

  Selenium::WebDriver::Firefox::Binary.path = '/Users/di/Applications/Firefox.app/Contents/MacOS/firefox-bin'
  profile = Selenium::WebDriver::Firefox::Profile.new
  profile['intl.accept_languages'] = 'en'

  Capybara::Selenium::Driver.new(app, {:browser => :firefox, :profile =>  profile})
end

Capybara::Screenshot.prune_strategy = :keep_last_run

Capybara.javascript_driver = :en_browser