A mid-sized fire broke out at the Denver Broncos’ stadium on Thursday. Firefighters quickly contained the blaze in the 300 level of Empower Field at Mile High, the Denver Fire Department said in a series of tweets. The fire burned dozens of seats and multiple suites. No injuries were reported. Firefighters said they were still working to determine the cause of the blaze, but it began in a construction area, according to the stadium’s Twitter account. The stadium’s sprinkler system was unable to contain the blaze, the fire department said. The Broncos have not played since Jan. 8, when they hos…