Make the crust: Preheat the oven to 350 degrees F. Brush an 8-inch springform pan with some of the melted butter. Mix the remaining butter with the graham cracker crumbs, sugar and salt in a bowl. Press the mixture into the bottom and up the sides of the prepared pan. Bake until golden brown, about 15 minutes. Cool completely on a rack.
Make the filling: Halve and pit the avocados, then scoop out the flesh and chop. Transfer to a bowl, add the cream cheese and beat with a mixer on medium speed until smooth. Add the condensed milk, lime and lemon juices, and the salt and beat until fluffy, scraping the bowl as needed. Pour the filling into the crust, press a piece of plastic wrap directly on the surface and chill at least 4 hours.
Remove the springform ring and slice the pie. Garnish with whipped cream or confectioners' sugar, if desired, and serve immediately (the top will start to brown as the pie sits).