​As long a there is a full mortar joint the face shell thickness on both sides of the wall there should be no question about the overall fire rating of the wall. Consider that the entire wall is FILLED with joints filled with mortar the face shell thickness of the wall. The joint at the top of the wall would be no different.

Most of the energy lost in a home is through the infiltration of outside air into the house, the hot water heating system, the efficiency of the AC unit, heat gain through the ceiling from the attic space and through the windows. The opaque exterior walls really do not contribute that significantly to the heat gain/loss, usually in the range of 6% – 10%. That being said, a modest amount of well-placed insulation can pay for itself in 2 – 3 years of energy savings. 
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The most common type of masonry insulation is foil backed paper stapled between the furring strips. This adds about an R3 of insulation and is the minimum amount recommended. The great thing about Florida is your temperature gradient is, by and large, small – usually less than 20 degF and for much of the year less than 5 degF. The maximum insulation recommended anywhere in Florida is ¾” Poly Iso with a foil backing. The non-foil side is glued or attached directly to the masonry and the furring strips go directly over the foil backing creating a ¾” reflective air space between the foil and the drywall. This arrangement gives you an approx. R7.8. You might want to use this on a large expanse of west facing wall where the heat gain in the afternoon can be more intense. 
Any more insulation than the R7.8 is a dead loss money wise and will NEVER pay for itself in energy savings. You can prove this to yourself by simply running the analysis in Comcheck or energy gauge software. You will quickly see that the exterior walls are not where your energy loss occurs.