I'm in actually in Chrome. I get the following error when running 90s.dev on that getting started page:
process.ts:159 SecurityError: Failed to execute 'showDirectoryPicker' on 'Window': Cross origin sub frames aren't allowed to show a file picker. at askdir (process.ts:154:37) at port.onmessage (rpc.ts:103:10) askdir @ process.ts:159 await in askdir port.onmessage @ rpc.ts:103 filer.app.js:129 Failed to get folder: error, cancelled, or using Firefox.
That's a new error, and it's because I changed some internals[1]. For now, you'll have to open the link[2] in a new tab and do the steps there.
[1] For about a week (and at the time of posting) the app was on 90s.dev/os/ to avoid this exact CORS error. But I got tired of how it forced me to name my github repos or pay for my own server, so I moved it back to os.90s.dev, which has the one downside of the error you ran into.
[2] https://os.90s.dev/#sys/apps/filer.app.js