Specify browser location for wct-local test

Question

The Polymer Starter Kit contains web-component-tester for unit testing your custom elements. In wct.conf.json you can specify the browsers to run the tests in.

{
  "suites": ["app/test"],
  "plugins": {
    "local": {
    "browsers": ["firefox"]
  }
}

Unfortunately, when I launch the tests through gulp, wct is not able to find my local Firefox. I read at GitHub that wct uses launchpad to detect local browsers. Afaik, on a Windows machine, launchpad searches for Firefox in the default directory "C:\Program Files\Mozilla Firefox". But I installed Firefox in a different directory.

Is there any way to configure the path to my Firefox dir in wct.conf.json?