# File lib/selenium/webdriver/firefox/profile.rb, line 92
        def []=(key, value)
          unless VALID_PREFERENCE_TYPES.any? { |e| value.kind_of? e }
            raise TypeError, "expected one of #{VALID_PREFERENCE_TYPES.inspect}, got #{value.inspect}:#{value.class}"
          end

          if value.kind_of?(String) && Util.stringified?(value)
            raise ArgumentError, "preference values must be plain strings: #{key.inspect} => #{value.inspect}"
          end

          @additional_prefs[key.to_s] = value
        end