diff --git a/dom/tests/mochitest/general/test_domWindowUtils.html b/dom/tests/mochitest/general/test_domWindowUtils.html
index 82d5fd276319..554a1af2b735 100644
--- a/dom/tests/mochitest/general/test_domWindowUtils.html
+++ b/dom/tests/mochitest/general/test_domWindowUtils.html
@@ -10,7 +10,7 @@
       padding: 0;
       margin: 0;
     }
-    
+
     div.test {
       position: absolute;
       height: 10px;
@@ -77,7 +77,7 @@ async function testElementFromPoint() {
     let [x, y, ignoreScroll, flushLayout] = testData[i][0];
     let moveData = testData[i][1];
     let expected = testData[i][2];
-    
+
     if (moveData) {
       let moveEl = moveData[1];
       let [moveX, moveY] = moveData[0];
@@ -106,6 +106,11 @@ async function testHandlingUserInput() {
       eventName: "click",
       result: true,
     },
+    {
+      eventName: "auxclick",
+      button: 1,
+      result: true,
+    },
     {
       eventName: "mousemove",
       result: false,
@@ -128,23 +133,25 @@ async function testHandlingUserInput() {
     },
   ];
 
-  for (let i=0; i<data.length; ++i) {
+  for (const {eventName, result, button} of data) {
     let eventPromise = new Promise(resolve => {
-      document.addEventListener(data[i].eventName, function() {
-        is(domWindowUtils.isHandlingUserInput, data[i].result,
-           "isHandlingUserInput should be " + data[i].result);
+      document.addEventListener(eventName, function() {
+        is(domWindowUtils.isHandlingUserInput, result,
+           `isHandlingUserInput should be ${result} for ${eventName}`);
 
         SimpleTest.executeSoon(resolve);
       }, {once: true});
     });
 
     SimpleTest.executeSoon(function() {
-      if (data[i].eventName == "click") {
+      if (eventName == "click") {
         synthesizeMouseAtCenter(document.body, {});
-      } else if (data[i].eventName.indexOf("key") == 0) {
-        synthesizeKey("VK_A", { type: data[i].eventName });
+      } else if (eventName == "auxclick" && button) {
+        synthesizeMouseAtCenter(document.body, { button });
+      } else if (eventName.startsWith("key")) {
+        synthesizeKey("VK_A", { type: eventName });
       } else {
-        synthesizeMouseAtCenter(document.body, { type: data[i].eventName });
+        synthesizeMouseAtCenter(document.body, { type: eventName });
       }
     });
 
diff --git a/testing/mochitest/tests/SimpleTest/EventUtils.js b/testing/mochitest/tests/SimpleTest/EventUtils.js
index 5387708f5288..968a7dfabd38 100644
--- a/testing/mochitest/tests/SimpleTest/EventUtils.js
+++ b/testing/mochitest/tests/SimpleTest/EventUtils.js
@@ -154,7 +154,8 @@ function computeButton(aEvent) {
 }
 
 function sendMouseEvent(aEvent, aTarget, aWindow) {
-  if (!['click', 'contextmenu', 'dblclick', 'mousedown', 'mouseup', 'mouseover', 'mouseout'].includes(aEvent.type)) {
+  if (!['click', 'contextmenu', 'dblclick', 'mousedown', 'mouseup', 'mouseover',
+        'mouseout'].includes(aEvent.type)) {
     throw new Error("sendMouseEvent doesn't know about event type '" + aEvent.type + "'");
   }