Preheat oven to 350 degrees with a rack in lower third. Butter a 9-by-5-inch loaf pan; line with parchment, leaving a 1-inch overhang on long sides. In a bowl, combine flour, brown sugar, cinnamon, and salt. Stir in butter and pecans until small clumps form and mixture is evenly moistened. (Streusel can be made ahead and refrigerated, covered, up to 3 days.)