Does this work on Firefox or Safari? You say “Firefox or Safari fall back to a virtual filesystem” but somehow I can’t get this fallback to work. When I click New Project I just get a reminder to use Chrome or Edge. Is there a button to press to trigger the fallback?

I'm on Safari, and it seems you can open your local LaTeX file to use the tool. It's an “Open file” button at the top.

Although I agree that it would be cool to have some hello-world example pre-opened.