Earlier quoted context omitted.
I tried quickly the app builder with Firefox and it seemed to mostly work fine. I would bet it's just that the devs use chrome and didn't have time to test extensively all their advanced features on all browsers.
What advanced features would be supported by a 3 year old Safari, a 4 year old Chrome, but not by Firefox? It's a kind of laziness that does the open web great harm by putting unnecessary barriers in front of Firefox users when they shouldn't be there. Feature detection for any newer APIs is far better. If you know you rely on a particular API set then you can test for precisely that. Otherwise let browsers be instea…
/s if you didn realize