diff --git a/gfx/layers/apz/test/mochitest/apz_test_utils.js b/gfx/layers/apz/test/mochitest/apz_test_utils.js
index a329c94adc72..b9bf8221fe82 100644
--- a/gfx/layers/apz/test/mochitest/apz_test_utils.js
+++ b/gfx/layers/apz/test/mochitest/apz_test_utils.js
@@ -378,7 +378,7 @@ async function waitUntilApzStable() {
     // it must be totally self-contained to be shipped over to the parent process.
     /* eslint-env mozilla/frame-script */
     function parentProcessFlush() {
-      addMessageListener("apz-flush", function() {
+      function apzFlush() {
         const {Services} = ChromeUtils.import("resource://gre/modules/Services.jsm");
         var topWin = Services.wm.getMostRecentWindow("navigator:browser");
         if (!topWin) {
@@ -409,13 +409,23 @@ async function waitUntilApzStable() {
         // Flush APZ repaints, but wait until all the pending paints have been
         // sent.
         flushRepaint();
-      });
+      }
+      function cleanup() {
+        removeMessageListener("apz-flush", apzFlush);
+        removeMessageListener("cleanup", cleanup);
+      }
+      addMessageListener("apz-flush", apzFlush);
+      addMessageListener("cleanup", cleanup);
     }
 
     // This is the first time waitUntilApzStable is being called, do initialization
     if (typeof waitUntilApzStable.chromeHelper == "undefined") {
       waitUntilApzStable.chromeHelper = SpecialPowers.loadChromeScript(parentProcessFlush);
-      ApzCleanup.register(() => { waitUntilApzStable.chromeHelper.destroy(); });
+      ApzCleanup.register(() => {
+        waitUntilApzStable.chromeHelper.sendSyncMessage("cleanup", null);
+        waitUntilApzStable.chromeHelper.destroy();
+        delete waitUntilApzStable.chromeHelper;
+      });
     }
 
     // Actually trigger the parent-process flush and wait for it to finish
diff --git a/testing/specialpowers/content/SpecialPowersObserverAPI.js b/testing/specialpowers/content/SpecialPowersObserverAPI.js
index 8c591aeca719..44179f3d84e4 100644
--- a/testing/specialpowers/content/SpecialPowersObserverAPI.js
+++ b/testing/specialpowers/content/SpecialPowersObserverAPI.js
@@ -466,7 +466,7 @@ SpecialPowersObserverAPI.prototype = {
         }
 
         // Setup a chrome sandbox that has access to sendAsyncMessage
-        // and addMessageListener in order to communicate with
+        // and {add,remove}MessageListener in order to communicate with
         // the mochitest.
         let systemPrincipal = Services.scriptSecurityManager.getSystemPrincipal();
         let sandboxOptions = Object.assign({wantGlobalProperties: ["ChromeUtils"]},
@@ -481,6 +481,14 @@ SpecialPowersObserverAPI.prototype = {
         sb.addMessageListener = (name, listener) => {
           this._chromeScriptListeners.push({ id, name, listener });
         };
+        sb.removeMessageListener = (name, listener) => {
+          let index = this._chromeScriptListeners.findIndex(function(obj) {
+            return obj.id == id && obj.name == name && obj.listener == listener;
+          });
+          if (index >= 0) {
+            this._chromeScriptListeners.splice(index, 1);
+          }
+        };
         sb.browserElement = aMessage.target;
 
         // Also expose assertion functions