Preheat oven to 300 degrees. Sift together flour and salt into a medium bowl. Beat butter with a mixer on medium-high speed until fluffy, 3 to 5 minutes, scraping down sides of bowl with a rubber spatula as needed. Add sugar gradually, beating between additions; continue to beat until mixture is very pale, about 2 minutes total. Reduce speed to low, and add flour mixture; beat until just combined.
Combine apricots, cranberries, candied citron, currants, and whiskey or brandy in a bowl. Let dried fruit macerate at least 2 hours and up to 8 hours; drain.
Add lemon zest to the butter with the sugar. After beating in the flour, mix in drained fruit. Form dough into a 10 1/2-inch log, then a triangle with 2 1/2-inch sides. (Refrigerate dough briefly if too soft to form.) Refrigerate until firm, about 1 hour; cut into 1/2-inch-thick slices.
Bake on parchment-lined baking sheets until firm and just golden on edges, about 35 minutes.