I was able to load your model with FF63 and the following glTFViewer which internally used three.js. Have you tried to to load the model on a different computer with FF63?
Just on a Android with FF63: failed. At work, I can’t install anything and FF60ESR works and at home, I just have 1 PC (FF63: failed). How does it do on your FF63?
If your reported display problem is actually a browser bug, it’s necessary to isolate the issue as far as possible. A small example is helpful in this context.