It's just the type tester element which is not accessible with browsers other than Safari or Chrome, because there's quite a lot of CSS trickery/hacks to make it work which didn't work in Firefox.

As mentioned in my other comment, due to the target audience it was decided that it wasn't worth the investment to get it working on other browsers right now, but the rest of the site should still work.

Not React's fault, just a lack of time/budget and a lack of native browser support for advanced typography.

False dichotomy. OP could have tested for FF if he wanted to, irrespective of the framework he has chosen.

Personally, I develop Firefox-first and then work on compatibility for lesser browsers like Chrome and Safari.

Of course things have been complicated a bit now that Firefox Focus is out and uses Webkit...