diff --git a/toolkit/devtools/server/actors/webapps.js b/toolkit/devtools/server/actors/webapps.js
index f8d8d91e38e..6e30ffcc2e4 100644
--- a/toolkit/devtools/server/actors/webapps.js
+++ b/toolkit/devtools/server/actors/webapps.js
@@ -565,12 +565,10 @@ WebappsActor.prototype = {
     }
 
     if (this._isAppAllowedForManifest(app.manifestURL)) {
-      let deferred = promise.defer();
-      reg.getManifestFor(manifestURL, function (manifest) {
+      return reg.getManifestFor(manifestURL).then(function (manifest) {
         app.manifest = manifest;
-        deferred.resolve({app: app});
+        return {app: app};
       });
-      return deferred.promise;
     }
     return { error: "forbidden" };
   },