Hi Pat, welcome to the forum.
Records for the Firefighting Platoons are mostly non existant, especialy for those units that were in Northern France. Most often the platoons would be under command of the Engineer Section of one of the Service Force Base Sections. Sometimes a numbered platoon will be found in a troop list of a Base Section if the list is included in the availible records. In thousands of pages of a Base Section histories, there will just be a mention like '1 section sent to depot' or '1 platoon assigned to port area' with no mention of the unit number. So it`s very hard to even find where a unit was.
But there may be another way to find where your dad was. In your post you mention letters your dad recieved. Do you have any letters he sent or recieved when he was in France? If you do, look for APO numbers in the addresses or on envelopes. We might be able to find where that Army Post Office was located.
The attached page is the ENTIRE firefighting plan in the Engineer Plan for Operation Neptune, the Normandy Invasion. It shows you the typical mentions of the FF Platoons.
Neptune Fire Fighting Plan0001.pdf
6 sections, probably about 40 men for the whole invasion force.