diff --git a/core/flyout_vertical.js b/core/flyout_vertical.js
index 78057a643..aa799123d 100644
--- a/core/flyout_vertical.js
+++ b/core/flyout_vertical.js
@@ -212,8 +212,9 @@ Blockly.VerticalFlyout.prototype.wheel_ = function(e) {
   var delta = e.deltaY;
 
   if (delta) {
-    if (goog.userAgent.GECKO) {
-      // Firefox's deltas are a tenth that of Chrome/Safari.
+    // Firefox's mouse wheel deltas are a tenth that of Chrome/Safari.
+    // DeltaMode is 1 for a mouse wheel, but not for a trackpad scroll event
+    if (goog.userAgent.GECKO && (e.deltaMode === 1)) {
       delta *= 10;
     }
     var metrics = this.getMetrics_();