--- a/testing/mochitest/tests/Harness_sanity/SpecialPowersLoadChromeScript.js
+++ b/testing/mochitest/tests/Harness_sanity/SpecialPowersLoadChromeScript.js
@@ -4,8 +4,12 @@ addMessageListener("foo", function (mess
   sendAsyncMessage("bar", message);
 });
 
 addMessageListener("valid-assert", function (message) {
   assert.ok(true, "valid assertion");
   assert.equal(1, 1, "another valid assertion");
   sendAsyncMessage("valid-assert-done");
 });
+
+addMessageListener("sync-message", () => {
+  return "Received a synchronous message.";
+});

--- a/testing/mochitest/tests/Harness_sanity/test_SpecialPowersLoadChromeScript.html
+++ b/testing/mochitest/tests/Harness_sanity/test_SpecialPowersLoadChromeScript.html
@@ -28,12 +28,22 @@ function checkAssert() {
 }
 
 function endOfTest() {
   script.destroy();
   SimpleTest.finish();
 }
 
 script.sendAsyncMessage("foo", MESSAGE);
+
+/*
+ * [0][0] is because we're using one real message listener in SpecialPowersObserverAPI.js
+ * and dispatching that to multiple _chromeScriptListeners. The outer array comes
+ * from the message manager since there can be multiple real listeners. The inner
+ * array is for the return values of _chromeScriptListeners.
+ */
+is(script.sendSyncMessage("sync-message")[0][0], "Received a synchronous message.",
+   "Check sync return value");
+
 </script>
 </pre>
 </body>
 </html>

--- a/testing/specialpowers/content/SpecialPowersObserverAPI.js
+++ b/testing/specialpowers/content/SpecialPowersObserverAPI.js
@@ -488,20 +488,19 @@ SpecialPowersObserverAPI.prototype = {
         }
         return undefined;	// See comment at the beginning of this function.
       }
 
       case "SPChromeScriptMessage": {
         let id = aMessage.json.id;
         let name = aMessage.json.name;
         let message = aMessage.json.message;
-        this._chromeScriptListeners
-            .filter(o => (o.name == name && o.id == id))
-            .forEach(o => o.listener(message));
-        return undefined;	// See comment at the beginning of this function.
+        return this._chromeScriptListeners
+                   .filter(o => (o.name == name && o.id == id))
+                   .map(o => o.listener(message));
       }
 
       case "SPImportInMainProcess": {
         var message = { hadError: false, errorMessage: null };
         try {
           Components.utils.import(aMessage.data);
         } catch (e) {
           message.hadError = true;

--- a/testing/specialpowers/content/specialpowersAPI.js
+++ b/testing/specialpowers/content/specialpowersAPI.js
@@ -486,16 +486,21 @@ SpecialPowersAPI.prototype = {
         );
       },
 
       sendAsyncMessage: (name, message) => {
         this._sendSyncMessage("SPChromeScriptMessage",
                               { id: id, name: name, message: message });
       },
 
+      sendSyncMessage: (name, message) => {
+        return this._sendSyncMessage("SPChromeScriptMessage",
+                                     { id, name, message });
+      },
+
       destroy: () => {
         listeners = [];
         this._removeMessageListener("SPChromeScriptMessage", chromeScript);
         this._removeMessageListener("SPChromeScriptAssert", chromeScript);
       },
 
       receiveMessage: (aMessage) => {
         let messageId = aMessage.json.id;