Gemfile

source "http://www.rubygems.org"

gem "selenium-webdriver"

repro.rb

## Have in a directory with Gemfile.
## Run `gem install bundler && bundle install` beforehand.
## Start a Marionette-enabled Selenium Server and change server_url to match (line 8).
## Then run this script with `bundle exec ruby test_script.rb`.
require "selenium/webdriver"

def server_url
  "http://127.94.0.1:4444/wd/hub"
end

def caps
caps = Selenium::WebDriver::Remote::Capabilities.firefox marionette: true
end

driver = Selenium::WebDriver.for :remote, :url => server_url, :desired_capabilities => caps

driver.navigate.to "http://google.com"
driver.find_element :tag_name, "h1"
driver.execute_script "window.open()"

puts driver.window_handles
driver.quit