--- a/testing/specialpowers/content/SpecialPowersAPI.jsm
+++ b/testing/specialpowers/content/SpecialPowersAPI.jsm
@@ -1544,69 +1544,57 @@ class SpecialPowersAPI extends JSWindowA
       });
     }
 
     // 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._extensionListeners.delete(listener);
-    });
 
     handler = Cu.waiveXrays(handler);
     ext = Cu.waiveXrays(ext);
 
     let sp = this;
     let state = "uninitialized";
     let extension = {
       get state() { return state; },
 
       startup() {
         state = "pending";
-        sp.sendAsyncMessage("SPStartupExtension", {id});
-        return startupPromise;
+        return sp.sendQuery("SPStartupExtension", {id}).then(
+          () => {
+            state = "running";
+          }, () => {
+            state = "failed";
+            sp._extensionListeners.delete(listener);
+            return Promise.reject("startup failed");
+          });
       },
 
       unload() {
         state = "unloading";
-        sp.sendAsyncMessage("SPUnloadExtension", {id});
-        return unloadPromise;
+        return sp.sendQuery("SPUnloadExtension", {id}).finally(() => {
+          sp._extensionListeners.delete(listener);
+          state = "unloaded";
+        });
       },
 
       sendMessage(...args) {
         sp.sendAsyncMessage("SPExtensionMessage", {id, args});
       },
     };
 
     this.sendAsyncMessage("SPLoadExtension", {ext, id});
 
     let listener = (msg) => {
       if (msg.data.id == id) {
-        if (msg.data.type == "extensionStarted") {
-          state = "running";
-          resolveStartup();
-        } else if (msg.data.type == "extensionSetId") {
+        if (msg.data.type == "extensionSetId") {
           extension.id = msg.data.args[0];
           extension.uuid = msg.data.args[1];
-        } else if (msg.data.type == "extensionFailed") {
-          state = "failed";
-          rejectStartup("startup failed");
-        } else if (msg.data.type == "extensionUnloaded") {
-          this._extensionListeners.delete(listener);
-          state = "unloaded";
-          resolveUnload();
         } else if (msg.data.type in handler) {
           handler[msg.data.type](...Cu.cloneInto(msg.data.args, this.contentWindow));
         } else {
           dump(`Unexpected: ${msg.data.type}\n`);
         }
       }
     };
 

--- a/testing/specialpowers/content/SpecialPowersAPIParent.jsm
+++ b/testing/specialpowers/content/SpecialPowersAPIParent.jsm
@@ -705,17 +705,17 @@ class SpecialPowersAPIParent extends JSW
           // is a MockExtension.
           this.sendAsyncMessage("SPExtensionMessage", {id, type: "extensionSetId", args: [ext.id, ext.uuid]});
         });
 
         // Make sure the extension passes the packaging checks when
         // they're run on a bare archive rather than a running instance,
         // as the add-on manager runs them.
         let extensionData = new ExtensionData(extension.rootURI);
-        extensionData.loadManifest().then(
+        return extensionData.loadManifest().then(
           () => {
             return extensionData.initAllLocales().then(() => {
               if (extensionData.errors.length) {
                 return Promise.reject("Extension contains packaging errors");
               }
               return undefined;
             });
           },
@@ -725,43 +725,37 @@ class SpecialPowersAPIParent extends JSW
             // case.
           }
         ).then(async () => {
           // browser tests do not call startup in ExtensionXPCShellUtils or MockExtension,
           // in that case we have an ID here and we need to set the override.
           if (extension.id) {
             await ExtensionTestCommon.setIncognitoOverride(extension);
           }
-          return extension.startup();
-        }).then(() => {
-          this.sendAsyncMessage("SPExtensionMessage", {id, type: "extensionStarted", args: []});
-        }).catch(e => {
-          dump(`Extension startup failed: ${e}\n${e.stack}`);
-          this.sendAsyncMessage("SPExtensionMessage", {id, type: "extensionFailed", args: []});
+          return extension.startup().then(() => {}, e => {
+            dump(`Extension startup failed: ${e}\n${e.stack}`);
+            throw e;
+          });
         });
-        return undefined;
       }
 
       case "SPExtensionMessage": {
         let id = aMessage.data.id;
         let extension = this._extensions.get(id);
         extension.testMessage(...aMessage.data.args);
         return undefined;
       }
 
       case "SPUnloadExtension": {
         let id = aMessage.data.id;
         let extension = this._extensions.get(id);
         this._extensions.delete(id);
-        let done = async () => {
-          await extension._uninstallPromise;
-          this.sendAsyncMessage("SPExtensionMessage", {id, type: "extensionUnloaded", args: []});
-        };
-        extension.shutdown().then(done, done);
-        return undefined;
+        return extension.shutdown().then(() => {
+          return extension._uninstallPromise;
+        });
       }
 
       case "Spawn": {
         let {browsingContext, task, args, caller} = aMessage.data;
 
         let spParent = browsingContext.currentWindowGlobal.getActor("SpecialPowers");
 
         let taskId = nextTaskID++;

--- a/toolkit/components/extensions/MessageChannel.jsm
+++ b/toolkit/components/extensions/MessageChannel.jsm
@@ -378,17 +378,17 @@ class ResponseManager extends FilteringM
    *
    * @param {string} messageName
    *     The internal message name for which to unregister the handler.
    * @param {object} handler
    *     The handler object to unregister.
    */
   removeHandler(messageName, handler) {
     if (DEBUG && this.handlers.get(messageName) !== handler) {
-      throw new Error(`Attempting to remove unexpected response handler for ${messageName}`);
+      Cu.reportError(`Attempting to remove unexpected response handler for ${messageName}`);
     }
     this.handlers.delete(messageName);
   }
 }
 
 /**
  * Manages mappings of message managers to their corresponding message
  * brokers. Brokers are lazily created for each message manager the