Preparation Melt butter in a small saucepan over medium heat. Let cool slightly. Crumble graham crackers into a food processor. Add melted butter, sugar, and 1/4 tsp. salt and process until mixture resembles wet sand. Transfer mixture to a shallow 9"-diameter pie dish. Using the bottom of a measuring cup, press crust firmly and evenly across bottom and up sides of dish. Freeze 20 minutes. Meanwhile, whisk cream in a medium bowl until stiff peaks form, about 4 minutes. Finely grate zest from 1 lime half into a small bowl; set aside. Squeeze juice from all of the limes into a fine-mesh sieve set over a glass measuring cup (you should have about 1/2 cup juice). Pour lime juice into another medium bowl. Add condensed milk, tequila, and remaining 1/4 tsp. salt and whisk until mixture is smooth. Gently fold in half of the whipped cream just to combine, then add the remaining whipped cream and gently fold until no streaks remain (be careful not to overmix; you want to keep as much air in the whipped cream as possible). Remove pie dish from the freezer and scrape filling into crust. Use a spoon to gently spread filling evenly all the way to the edges. Sprinkle with reserved lime zest and freeze until filling is set, at least 8 hours. Do Ahead: Pie can be made 3 days ahead. Cover; keep chilled. Let soften in fridge 30 minutes before slicing.