fix ruby error for program

fix this error on ruby C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/webdriver/firefox/[login to view URL]:in `path': Could not find Firefox binary (os=windows). Make sure Firefox is installed or set the path manually with Selenium::WebDriver::Firefox::Binary.path= (Selenium::WebDriver::Error::WebDriverError) from C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/webdriver/firefox/[login to view URL]:in `execute' from C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/webdriver/firefox/[login to view URL]:in `start_with' from C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/webdriver/firefox/[login to view URL]:in `start' from C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/webdriver/firefox/[login to view URL]:in `block in launch' from C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/webdriver/common/[login to view URL]:in `locked' from C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/webdriver/firefox/[login to view URL]:in `launch' from C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/webdriver/firefox/[login to view URL]:in `initialize' from C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/webdriver/common/[login to view URL]:in `new' from C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/webdriver/common/[login to view URL]:in `for' from C:/Ruby200/lib/ruby/gems/2.0.0/gems/selenium-webdriver-2.48.1/lib/selenium/[login to view URL]:in `for' from C:/Ruby200/lib/ruby/gems/2.0.0/gems/watir-webdriver-0.9.1/lib/watir-webdriver/[login to view URL]:in `initialize' from [login to view URL]:in `new' from [login to view URL]:in `<main>'