--- a/testing/specialpowers/content/specialpowersAPI.js
+++ b/testing/specialpowers/content/specialpowersAPI.js
@@ -1940,31 +1940,47 @@ SpecialPowersAPI.prototype = {
     this.notifyObserversInParentProcess(null, "browser:purge-domain-data", "example.com");
   },
 
   cleanUpSTSData: function(origin, flags) {
     return this._sendSyncMessage('SPCleanUpSTSData', {origin: origin, flags: flags || 0});
   },
 
   _nextExtensionID: 0,
+  _extensionListeners: null,
+
   loadExtension: function(ext, handler) {
+    if (this._extensionListeners == null) {
+      this._extensionListeners = new Set();
+
+      this._addMessageListener("SPExtensionMessage", msg => {
+        for (let listener of this._extensionListeners) {
+          try {
+            listener(msg);
+          } catch (e) {
+            Cu.reportError(e);
+          }
+        }
+      });
+    }
+
     // Note, this is not the addon is as used by the AddonManager etc,
     // this is just an identifier used for specialpowers messaging
     // between this content process and the chrome process.
     let id = this._nextExtensionID++;
 
     let resolveStartup, resolveUnload, rejectStartup;
     let startupPromise = new Promise((resolve, reject) => {
       resolveStartup = resolve;
       rejectStartup = reject;
     });
     let unloadPromise = new Promise(resolve => { resolveUnload = resolve; });
 
     startupPromise.catch(() => {
-      this._removeMessageListener("SPExtensionMessage", listener);
+      this._extensionListeners.delete(listener);
     });
 
     handler = Cu.waiveXrays(handler);
     ext = Cu.waiveXrays(ext);
 
     let sp = this;
     let state = "uninitialized";
     let extension = {
@@ -1995,28 +2011,28 @@ SpecialPowersAPI.prototype = {
           state = "running";
           resolveStartup();
         } else if (msg.data.type == "extensionSetId") {
           extension.id = msg.data.args[0];
         } else if (msg.data.type == "extensionFailed") {
           state = "failed";
           rejectStartup("startup failed");
         } else if (msg.data.type == "extensionUnloaded") {
-          this._removeMessageListener("SPExtensionMessage", listener);
+          this._extensionListeners.delete(listener);
           state = "unloaded";
           resolveUnload();
         } else if (msg.data.type in handler) {
           handler[msg.data.type](...msg.data.args);
         } else {
           dump(`Unexpected: ${msg.data.type}\n`);
         }
       }
     };
 
-    this._addMessageListener("SPExtensionMessage", listener);
+    this._extensionListeners.add(listener);
     return extension;
   },
 
   invalidateExtensionStorageCache: function() {
     this.notifyObserversInParentProcess(null, "extension-invalidate-storage-cache", "");
   },
 
   allowMedia: function(window, enable) {