Untitled

require 'watir-webdriver'

profile = Selenium::WebDriver::Firefox::Profile.new
profile.log_file = "/tmp/firefox.log"

d = Watir::Browser.new :firefox, :profile => profile
d.goto "data:text/html;content-type=utf-8,#{URI.encode DATA.read}"
d.quit

Dir['/tmp/firefox*.log'].each do |file|
  p :file => file, :has_client_error => File.read(file).include?('oops')
end

# output:
#
# {:file=>"/tmp/firefox-1.log", :has_client_error=>false}
# {:file=>"/tmp/firefox.log", :has_client_error=>true}
#

__END__
<html>
  <body>
    <script>
      throw new Error('oops');
    </script>
  </body>
</html>