The Selenium system tests do not use the latest version of Firefox (53) but an older one: Firefox 47.

Using this version it's impossible to login to admin panel, the application hangs on the "Loading the app..." screen. An error is visible in console:

SyntaxError: missing : after property id

Everything works fine on the latest Firefox.

It's necessary to upgrade the Firefox version to the latest one (together with a matching Selenium Server version).