# File lib/selenium/webdriver/firefox/profile.rb, line 83
        def []=(key, value)
          case value
          when String
            if Util.stringified?(value)
              raise ArgumentError, "preference values must be plain strings: #{key.inspect} => #{value.inspect}"
            end

            value = %{"#{value}"}
          when TrueClass, FalseClass, Integer, Float
            value = value.to_s
          else
            raise TypeError, "invalid preference: #{value.inspect}:#{value.class}"
          end

          @additional_prefs[key.to_s] = value
        end