diff --git a/testing/mochitest/browser-test.js b/testing/mochitest/browser-test.js
index 9d7c541020de..351ef4138549 100644
--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -477,6 +477,9 @@ function Tester(aTests, structuredLogger, aCallback) {
   );
   this.SimpleTest = simpleTestScope.SimpleTest;
 
+  window.SpecialPowers.SimpleTest = this.SimpleTest;
+  window.SpecialPowers.setAsDefaultAssertHandler();
+
   var extensionUtilsScope = {
     registerCleanupFunction: fn => {
       this.currentTest.scope.registerCleanupFunction(fn);
diff --git a/testing/mochitest/tests/Harness_sanity/test_SpecialPowersSandbox.html b/testing/mochitest/tests/Harness_sanity/test_SpecialPowersSandbox.html
index 6208a35af4dc..36c8280b684f 100644
--- a/testing/mochitest/tests/Harness_sanity/test_SpecialPowersSandbox.html
+++ b/testing/mochitest/tests/Harness_sanity/test_SpecialPowersSandbox.html
@@ -37,8 +37,11 @@ async function interceptDiagnostics(func) {
 }
 
 add_task(async function() {
+  const frameSrc = "https://example.com/tests/testing/mochitest/tests/Harness_sanity/file_spawn.html";
+  const subframeSrc = "https://example.org/tests/testing/mochitest/tests/Harness_sanity/file_spawn.html";
+
   let frame = document.getElementById("iframe");
-  frame.src = "https://example.com/tests/testing/mochitest/tests/Harness_sanity/file_spawn.html";
+  frame.src = frameSrc;
 
   await new Promise(resolve => {
     frame.addEventListener("load", resolve, {once: true});
@@ -59,13 +62,31 @@ add_task(async function() {
   // just need to make sure that the general functionality works as expected.
   let tests = {
     "SpecialPowers.spawn": () => {
-      return SpecialPowers.spawn(frame, [], () => {
+      return SpecialPowers.spawn(frame, [], async () => {
         Assert.equal(1, 2, "Thing");
         Assert.equal(1, 1, "Hmm");
         Assert.ok(true, "Yay.");
         Assert.ok(false, "Boo!.");
       });
     },
+    "SpecialPowers.spawn-subframe": () => {
+      return SpecialPowers.spawn(frame, [subframeSrc], async src => {
+        let frame = this.content.document.createElement("iframe");
+        frame.src = src;
+        this.content.document.body.appendChild(frame);
+
+        await new Promise(resolve => {
+          frame.addEventListener("load", resolve, { once: true });
+        });
+
+        await SpecialPowers.spawn(frame, [], () => {
+          Assert.equal(1, 2, "Thing");
+          Assert.equal(1, 1, "Hmm");
+          Assert.ok(true, "Yay.");
+          Assert.ok(false, "Boo!.");
+        });
+      });
+    },
     "SpecialPowers.loadChromeScript": async () => {
       let script = SpecialPowers.loadChromeScript(() => {
         this.addMessageListener("ping", () => "pong");
diff --git a/testing/mochitest/tests/SimpleTest/SimpleTest.js b/testing/mochitest/tests/SimpleTest/SimpleTest.js
index c59e6b237a04..05362cf1b19c 100644
--- a/testing/mochitest/tests/SimpleTest/SimpleTest.js
+++ b/testing/mochitest/tests/SimpleTest/SimpleTest.js
@@ -228,6 +228,8 @@ SimpleTest._inChaosMode = false;
 SimpleTest.expected = 'pass';
 SimpleTest.num_failed = 0;
 
+SpecialPowers.setAsDefaultAssertHandler();
+
 function usesFailurePatterns() {
   return Array.isArray(SimpleTest.expected);
 }
diff --git a/testing/specialpowers/content/SpecialPowersChild.jsm b/testing/specialpowers/content/SpecialPowersChild.jsm
index e51294e9f27b..5766721a2813 100644
--- a/testing/specialpowers/content/SpecialPowersChild.jsm
+++ b/testing/specialpowers/content/SpecialPowersChild.jsm
@@ -285,9 +285,7 @@ class SpecialPowersChild extends JSWindowActorChild {
           // An assertion has been done in a mochitest chrome script
           let { name, passed, stack, diag } = message.data;
 
-          let SimpleTest =
-            this.contentWindow && this.contentWindow.wrappedJSObject.SimpleTest;
-
+          let { SimpleTest } = this;
           if (SimpleTest) {
             SimpleTest.record(
               passed,
@@ -1661,6 +1659,7 @@ class SpecialPowersChild extends JSWindowActorChild {
       args,
       task: String(task),
       caller: SpecialPowersSandbox.getCallerInfo(Components.stack.caller),
+      hasHarness: typeof this.SimpleTest === "object",
     });
   }
 
@@ -1692,6 +1691,21 @@ class SpecialPowersChild extends JSWindowActorChild {
     return sb.execute(task, args, caller);
   }
 
+  get SimpleTest() {
+    return this._SimpleTest || this.contentWindow.wrappedJSObject.SimpleTest;
+  }
+  set SimpleTest(val) {
+    this._SimpleTest = val;
+  }
+
+  /**
+   * Sets this actor as the default assertion result handler for tasks
+   * which originate in a window without a test harness.
+   */
+  setAsDefaultAssertHandler() {
+    this.sendAsyncMessage("SetAsDefaultAssertHandler");
+  }
+
   getFocusedElementForWindow(targetWindow, aDeep) {
     var outParam = {};
     Services.focus.getFocusedElementForWindow(targetWindow, aDeep, outParam);
diff --git a/testing/specialpowers/content/SpecialPowersParent.jsm b/testing/specialpowers/content/SpecialPowersParent.jsm
index d75d80d5c881..e36c2bd63eef 100644
--- a/testing/specialpowers/content/SpecialPowersParent.jsm
+++ b/testing/specialpowers/content/SpecialPowersParent.jsm
@@ -119,6 +119,10 @@ function doPrefEnvOp(fn) {
 // used to bounce assertion messages back down to the correct child.
 let nextTaskID = 1;
 
+// The default actor to send assertions to if a task originated in a
+// window without a test harness.
+let defaultAssertHandler;
+
 class SpecialPowersParent extends JSWindowActorParent {
   constructor() {
     super();
@@ -200,6 +204,10 @@ class SpecialPowersParent extends JSWindowActorParent {
   }
 
   uninit() {
+    if (defaultAssertHandler === this) {
+      defaultAssertHandler = null;
+    }
+
     var obs = Services.obs;
     obs.removeObserver(this._observer, "http-on-modify-request");
     this._registerObservers._topics.splice(0).forEach(element => {
@@ -1042,15 +1050,22 @@ class SpecialPowersParent extends JSWindowActorParent {
         });
       }
 
+      case "SetAsDefaultAssertHandler": {
+        defaultAssertHandler = this;
+        return undefined;
+      }
+
       case "Spawn": {
-        let { browsingContext, task, args, caller } = aMessage.data;
+        let { browsingContext, task, args, caller, hasHarness } = aMessage.data;
 
         let spParent = browsingContext.currentWindowGlobal.getActor(
           "SpecialPowers"
         );
 
         let taskId = nextTaskID++;
-        spParent._taskActors.set(taskId, this);
+        if (hasHarness) {
+          spParent._taskActors.set(taskId, this);
+        }
 
         return spParent
           .sendQuery("Spawn", { task, args, caller, taskId })
@@ -1083,9 +1098,10 @@ class SpecialPowersParent extends JSWindowActorParent {
 
       case "ProxiedAssert": {
         let { taskId, data } = aMessage.data;
-        let actor = this._taskActors.get(taskId);
 
+        let actor = this._taskActors.get(taskId) || defaultAssertHandler;
         actor.sendAsyncMessage("Assert", data);
+
         return undefined;
       }