This website is using some really recent web technologies which are still shaping up and their support differs in different browsers, so some people might be getting errors. There's not much I can do about it - we just have to wait till the technology is more settled.
For the moment the Mirage Editor as well as the entire website would work best as an app, similarly to Discord. But nobody would use it if it had to be installed. It works best on Chrome/Edge/Vivaldi, and I'm still improving Firefox support. I'll use this thread to post the most crucial updates.