If you missed the Tokyo firefly events in June, all is not lost. Along with an obon flower market, Setagaya hosts its firefly festival in mid-July in the grounds of Daikanyashiki, the house of a feudal lord during the Edo period which was designated an “Important Cultural Property” (重要文化財) in 1978.
Organisers release 3,000 fireflies in a temporary dome put over the car park at Daikanyashiki. The event times are 5pm to 9pm, but visitors are allowed into the dome after 7pm. Note that the Sagiso market (a species of orchid and official flower of Setagaya Ward) takes place in the grounds of the nearby Kamimachitenso Shrine between 1pm and 9pm. See here for suggestions on other places to enjoy the fireflies in and near Tokyo.