--- a/toolkit/mozapps/extensions/AddonManager.jsm
+++ b/toolkit/mozapps/extensions/AddonManager.jsm
@@ -2218,17 +2218,17 @@ var AddonManagerInternal = {
                                 Cr.NS_ERROR_INVALID_ARG);
 
   if (!aCallback || typeof aCallback != "function")
     throw Components.Exception("aCallback must be a function",
                                Cr.NS_ERROR_INVALID_ARG);
 
    this.getAddonByInstanceID(aInstanceID).then(wrapper => {
      if (!wrapper) {
-       throw Error("No addon matching instanceID:", aInstanceID.toString());
+       throw Error(`No addon matching instanceID: ${aInstanceID}`);
      }
      let addonId = wrapper.id;
      logger.debug(`Registering upgrade listener for ${addonId}`);
      this.upgradeListeners.set(addonId, aCallback);
    });
   },
 
   /**
@@ -2239,22 +2239,22 @@ var AddonManagerInternal = {
    */
   removeUpgradeListener(aInstanceID) {
     if (!aInstanceID || typeof aInstanceID != "symbol")
       throw Components.Exception("aInstanceID must be a symbol",
                                  Cr.NS_ERROR_INVALID_ARG);
 
     return this.getAddonByInstanceID(aInstanceID).then(addon => {
       if (!addon) {
-        throw Error("No addon for instanceID:", aInstanceID.toString());
+        throw Error(`No addon for instanceID: ${aInstanceID}`);
       }
       if (this.upgradeListeners.has(addon.id)) {
         this.upgradeListeners.delete(addon.id);
       } else {
-        throw Error("No upgrade listener registered for addon ID:", addon.id);
+        throw Error(`No upgrade listener registered for addon ID: ${addon.id}`);
       }
     });
   },
 
   /**
    * Installs a temporary add-on from a local file or directory.
    * @param  aFile
    *         An nsIFile for the file or directory of the add-on to be