"Modern HTML and JS APIs" were designed by Chrome for Chrome, so unsurprisingly work better on Chrome.
And Firefox solution is to ignore all developers that think this is a great platform compared to Play Store and App Store. Great way to lose market share.
I think you're justified in this case - this isn't about not supporting Chrome or the features it offers, just saying that it has some issues that make it not great as a browser for many end-users. This is one such case that hopefully has a resolution. But, as for any platform, I think it's fair to take advantage of additional things one platform gives you over the other. That's so long as you properly support the others still.
I use lots of modern APIs and CSS (on our own site) with no issues. I can only remember one site in the past month that had an issue in FF, and it was probably because of user agent checking. What API's are you referring to, and where did you come across them?
My new apps make use of modern HTML and JS APIs and I can't count on Firefox implementing them.