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.
In 1998 you had to be running Windows. Now you can run any OS, even if the browser you have to use for a certain site isn't your favorite. It's an improvement.
People have always wanted to use "the web" as an application platform. It didn't work that well in 1998, but it's pretty good now. Progress has been made.
edit: the site will now warn you if your browser is incompatible.