Firefly returns to Dover Thursday after a year off because of the COVID-19 pandemic. Photo by Firefly.   After taking a year off due to the pandemic, Firefly Music Festival is returning to the Woodlands at Dover, bringing a star-studded lineup headlined by Billie Eilish, The Killers, Tame Impala and Lizzo.  The event will begin Thursday, Sept. 23 and run …