--- a/gfx/layers/apz/test/mochitest/apz_test_utils.js
+++ b/gfx/layers/apz/test/mochitest/apz_test_utils.js
@@ -312,35 +312,41 @@ async function promiseApzFlushedRepaints
 //           passed exactly one argument, which will be the child window.
 // An example of an array is:
 //   aSubtests = [
 //     { 'file': 'test_file_name.html' },
 //     { 'file': 'test_file_2.html', 'prefs': [['pref.name', true], ['other.pref', 1000]], 'dp_suppression': false }
 //     { 'file': 'file_3.html', 'onload': function(w) { w.subtestDone(); } }
 //   ];
 //
-// Each subtest should call the subtestDone() function when it is done, to
-// indicate that the window should be torn down and the next text should run.
-// The subtestDone() function is injected into the subtest's window by this
+// Each subtest should call one of the subtestDone() or subtestFailed()
+// functions when it is done, to indicate that the window should be torn
+// down and the next test should run.
+// These functions are injected into the subtest's window by this
 // function prior to loading the subtest. For convenience, the |is| and |ok|
 // functions provided by SimpleTest are also mapped into the subtest's window.
 // For other things from the parent, the subtest can use window.opener.<whatever>
 // to access objects.
 function runSubtestsSeriallyInFreshWindows(aSubtests) {
   return new Promise(function(resolve, reject) {
     var testIndex = -1;
     var w = null;
 
     // If the "apz.subtest" pref has been set, only a single subtest whose name matches
     // the pref's value (if any) will be run.
     var onlyOneSubtest = SpecialPowers.getCharPref(
       "apz.subtest",
       /* default = */ ""
     );
 
+    function advanceSubtestExecutionWithFailure(msg) {
+      SimpleTest.ok(false, msg);
+      advanceSubtestExecution();
+    }
+
     function advanceSubtestExecution() {
       var test = aSubtests[testIndex];
       if (w) {
         // Run any cleanup functions registered in the subtest
         // Guard against the subtest not loading apz_test_utils.js
         if (w.ApzCleanup) {
           w.ApzCleanup.execute();
         }
@@ -412,16 +418,17 @@ function runSubtestsSeriallyInFreshWindo
         SpecialPowers.getDOMWindowUtils(window).respectDisplayPortSuppression(
           test.dp_suppression
         );
       }
 
       function spawnTest(aFile) {
         w = window.open("", "_blank");
         w.subtestDone = advanceSubtestExecution;
+        w.subtestFailed = advanceSubtestExecutionWithFailure;
         w.isApzSubtest = true;
         w.SimpleTest = SimpleTest;
         w.dump = function(msg) {
           return dump(aFile + " | " + msg);
         };
         w.is = function(a, b, msg) {
           return is(a, b, aFile + " | " + msg);
         };