Officials say it started around 6:19 Monday morning at 1311 South Main Street, a one-story commercial building that houses Discount Furniture, a discount furniture and mattress store.
Firefighters saw heavy smoke coming from the building, and began fighting the fire from inside. But about an hour later, the fire worsened and it became a defensive operation.
Part of the building has collapsed, but no other buildings were damaged. While the fire has been knocked down considerably, officials say it is still not under control.
No one is hurt, according to officials.
DEEP is on the scene due to the possibility of runoff into the Naugatuck River.
The Fire Marshal is also on scene and an investigation into the cause of the fire is now underway.