Web components setup does not work with firefox

[timonla]

The usage of web components introduced in #53 does not work in firefox. The custom elements get injected without any errors but the shadow root of these elements is null.

Even though we don't use the specific polyfill from the screenshot below, I suspect that it has something to do with some polyfill not loading before the custom elements are rendered.

https://lit-element.polymer-project.org/guide/use