Drone cannot show token in the WebUI in Firefox

I have a small problem that drone will not show my token in the WebUI in Firefox 53.0 (64-bit, Linux). It works fine in Chromium (Linux).

The browser console says:
TypeError: f.findDOMNode(…).showModal is not a function app.js:2061:719

This error is shown once when clicking on the “SHOW TOKEN” button, subsequent clicking produces not console errors but will still not show me the token.

Once I find out which drone version runs on our server (which is not apparent from the UI), I will add this here. Edit: it is 0.5