@ LcSummers
From a quick search on the development of the Firefly, it seems the Brits desired more stowage space for the 17 pounder ammo over the hull mg, thus eliminated the bow machine gunner position altogether for this purpose. The 17 pounder was much longer than the 75mm rounds carried by other sherman variants. Hope this helps.
Link to info source:
http://en.wikipedia.org/wiki/Sherman_Firefly