--- a/toolkit/content/widgets/spinner.js
+++ b/toolkit/content/widgets/spinner.js
@@ -295,21 +295,21 @@ function Spinner(props, context) {
             down: true,
             layerX: event.layerX,
             layerY: event.layerY
           };
           if (event.target == up) {
             // An "active" class is needed to simulate :active pseudo-class
             // because element is not focused.
             event.target.classList.add("active");
-            this._smoothScrollToIndex(index + 1);
+            this._smoothScrollToIndex(index - 1);
           }
           if (event.target == down) {
             event.target.classList.add("active");
-            this._smoothScrollToIndex(index - 1);
+            this._smoothScrollToIndex(index + 1);
           }
           if (event.target.parentNode == spinner) {
             // Listen to dragging events
             spinner.addEventListener("mousemove", this, { passive: true });
             spinner.addEventListener("mouseleave", this, { passive: true });
           }
           break;
         }