arcgis server - L.esri.featureLayer shows up and sometimes not

I'm new to the leaflet/esri subject so please forgive me if it's just a simple question.

I wrote my first webmap-app. It uses different sources. The main aim is to show two basemaps hosted by MapBox with an AGS layer coming from a different source.

Mapbox does not make any trouble. The problem is the AGS layer. Its address is:

http://geoportal1.stadt-koeln.de/ArcGIS/rest/services/Stadtplanthemen/MapServer/9

My script shows four different behaviours. I tested it with two different AG Servers:

1. 
   
   

   Test with http://geoportal1.stadt-koeln.de/ArcGIS/rest/services/Stadtplanthemen/MapServer/9 a: It shows up in IE10 correctly b: Not showing up in FF 33

   
   


2. 
   

   Test with the same script but a different AGS http://services.arcgis.com/rOo16HdIMeOBI4Mb/arcgis/rest/services/Heritage_Trees_Portland/FeatureServer/0 a: Shows up in IE 10 b: Shows up in FF 33

   
   

Well, what is the error in this script? The geoportal1's AGS seems to work properly. FF understands the script. But what's not working? If I use L.esri.dynamicMapLayer instead of L.esri.featureLayer the layer shows up. But for further uses I need the L.esri.featureLayer.

I also published it here:

https://github.com/Esri/esri-leaflet/issues/412

Hopefully someone can help!