Well, Forsythia blooms on wood that grew the previous year, so it depends on how much growing season remains in your particular climate. If there is time for a good growth of new stems, and time for those stems to ripen and form flower buds before frost, then prune now. If winter comes early where you are, then it may be better to do some selective pruning, just so it doesn't grab passers by, and wait for the hard pruning until after next year's bloom.

Just had a look at the other replies to your first question. I stand by mine, but if you like it as it is now why cut it back? The main reason fur cutting them back is to prevent them from getting straggly and as yours is compact missing year won't matter at all.