I'm sure I visited this tower at some point during my 20 years or so of living in Billerica but I can't confirm it. I headed over right at sunrise on Saturday to take a few pictures. Even being stealthy I was called out by someone living in the condo units nearby. They were okay with me taking pics.
From the Fire Tower website:
The Billerica fire tower was an existing structure of stone located on the grounds of the Maryknoll / St. Vincent's seminary off Wyman road. It was pressed into service by the state Department of Conservation in 1938 as a fire tower when the Chelmsford tower was destroyed in the huricane of 1938 It was used as a fire tower until a new Chelmsford tower could be rebuilt in 1939.
The tower site is now a horse paddock and the tower itself houses at least one horse!
Look closely and you can see him in my picture.