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.
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 instead of putting banners up recommending "best in IE". It just harms the web.
Making a static web page work on different browsers is much easier than making a web app work on all the browsers. For a web app, even 1% of the API doesn't work, the whole app is a failure. Can you accept an app only work 99% of the time?
I've no idea what makes it incompatible with Firefox, but without knowing the reason, it's an offence to call it laziness.
If they didn't bother documenting the compatibility issues, even if only so that future roadmaps can review and take stock of any changes in browser functionality down the road -- or to allow others to work on/around the problem, that's laziness and it's fair to call it out as such.
Personally, I would rather have a warning that FF "may" have issues, than no warning at all. Nothing is stopping a user form using FF if they want, and now they have a heads up.
As a long time open source user and contributor, I find it disappointing when some people can become entitled and complain about free software that doesn't exactly meet their expectations.
As mentioned elsewhere, the fact that it is open source means that anyone can potentially contribute... so if it is a large issue, then there is nothing stopping folks from trying to solve the problem themselves.
/s if you didn realize