--- a/toolkit/devtools/server/actors/webapps.js
+++ b/toolkit/devtools/server/actors/webapps.js
@@ -942,18 +942,17 @@ WebappsActor.prototype = {
 
     return {};
   },
 
   handleEvent: function (event) {
     let manifestURL;
     switch(event.type) {
       case "appwillopen":
-        let frame = event.target;
-        manifestURL = frame.getAttribute("mozapp")
+        manifestURL = event.detail.manifestURL;
 
         // Ignore the event if we already received an appwillopen for this app
         // (appwillopen is also fired when the app has been moved to background
         // and get back to foreground)
         if (this._openedApps.has(manifestURL)) {
           return;
         }
         this._openedApps.add(manifestURL);