NORTHERN LIBERTIES (WPVI) -- A firefighter is on the mend after being injured at a fire house in the Northern Liberties section of Philadelphia.It happened around 10:30 p.m. Wednesday at the station located at 4th and Girard streets.Police say a firefighter encountered a disoriented man who allegedly slashed his arm.He was taken to Aria Hospital campus for treatment with non-life threatening injuries.The suspect was arrested after the alleged incident.