--- a/testing/marionette/event.js
+++ b/testing/marionette/event.js
@@ -157,31 +157,31 @@ event.sendString = function(string, wind
  */
 event.sendKey = function(key, window = undefined) {
   let keyName = "VK_" + key.toUpperCase();
   event.synthesizeKey(keyName, {shiftKey: false}, window);
 };
 
 // TODO(ato): Unexpose this when action.Chain#emitMouseEvent
 // no longer emits its own events
-event.parseModifiers_ = function(event) {
+event.parseModifiers_ = function(modifiers) {
   let mval = 0;
-  if (event.shiftKey) {
+  if (modifiers.shiftKey) {
     mval |= Ci.nsIDOMNSEvent.SHIFT_MASK;
   }
-  if (event.ctrlKey) {
+  if (modifiers.ctrlKey) {
     mval |= Ci.nsIDOMNSEvent.CONTROL_MASK;
   }
-  if (event.altKey) {
+  if (modifiers.altKey) {
     mval |= Ci.nsIDOMNSEvent.ALT_MASK;
   }
-  if (event.metaKey) {
+  if (modifiers.metaKey) {
     mval |= Ci.nsIDOMNSEvent.META_MASK;
   }
-  if (event.accelKey) {
+  if (modifiers.accelKey) {
     if (navigator.platform.indexOf("Mac") >= 0) {
       mval |= Ci.nsIDOMNSEvent.META_MASK;
     } else {
       mval |= Ci.nsIDOMNSEvent.CONTROL_MASK;
     }
   }
   return mval;
 };