I won't bother trying it out if it forces me to use chromium
It seems that your browser doesn't support the new native filesystem API, please use any Chromium 86+ based browser like Chrome, Vivaldi, Edge, etc. Notice that the API doesn't support mobile browsers at the moment.
The dev team is working on replacing the filesystem API with another protocol supported by Firefox. That page is just a demo; you only need the support if you want to save your changes to the demo. You can try the desktop app via flathub.