class Selenium::WebDriver::Firefox::Profile

def []=(key, value)
  unless VALID_PREFERENCE_TYPES.any? { |e| value.is_a? e }
    raise TypeError, "expected one of #{VALID_PREFERENCE_TYPES.inspect}, got #{value.inspect}:#{value.class}"
  end
  if value.is_a?(String) && Util.stringified?(value)
    raise ArgumentError, "preference values must be plain strings: #{key.inspect} => #{value.inspect}"
  end
  @additional_prefs[key.to_s] = value
end