Is this supposed to work cross-browser?

I tried to follow instructions at https://github.com/firebase/quickstart-js/tree/master/messaging as closely as possible, but I still find the behavior of the messaging quickstart inconsistent -while it works in Chrome as well as FF, the subscription button doesn't show up in either browser. This is not a big issue, because the browser displays a popup asking user to subscribe. This popup needs to be manually selected in Firefox. I wasn't able to debug the inline script in `index.html` - it looks like it's not executed. Am I doing anything wrong?