Slightly embarrassing -- but the problem may be that the site isn't compatible with FireFox (at least pre-v63 FireFox). The site makes use of custom HTML elements which are not supported by default in pre-v63 FireFox (https://developer.mozilla.org/en-US/docs/Web/Web_Components/...). The good news is that all you need to do is to turn on a flag as set forth in the link provided. The bad news is that many other less tech-savvy people are going to be stymied by this.

edit: the site will now warn you if your browser is incompatible.