Although there is more than one possible cause of leaf drop, I would suggest checking the Ficus tree for the presence of fig whitefly. This insect has been present in south Florida for a few years but was discovered in Pinellas County this summer in St. Petersburg.

Like other kinds of whiteflies, fig whitefly feeds on plant sap. Ficus leaves that are infested with fig whitefly turn yellow and drop from the tree. Ficus trees without their leaves are one of the most obvious symptoms of a fig whitely infestation. Also, if a branch of an infested tree is shaken, the small, white gnat-like adult whiteflies will fly out.

Control of the fig whitefly may be difficult on large trees. If the tree is small enough to spray, you may want to try either a light horticultural oil spray or an insecticidal soap. Through coverage, along with repeat applications, is necessary for control.

More information on fig whitefly, including other control options, can be found here: http://mrec.ifas.ufl.edu/lso/IAWG/FIG/The%20Fig%20Whitefly.htm

http://collier.ifas.ufl.edu/CommHort/CommHortPubs/Fig%20whitefly%20defoliating%20weeping%20ficus%20trees%20and%20hedges.pdf

Of course, with the recent extreme cold many or most of leaves on your Ficus tree may have been damaged anyway. Freeze damaged leaves will usually turn brown within a few days and drop from the tree.