For the Firebase Studio app icon, edit .idx/icon.png to the image you want.
The 404 issue can be a lot of things, but mainly Next.js is unable to serve the route you’re requesting for one reason or another. The best place to start is with your server logs. Look for the output window (SHIFT+CTRL+U) and select “Previews” in the dropdown. Look through the output to see what might have gone wrong with the page request. Copy and paste the error into Gemini to help troubleshoot it if you don’t understand the message.
Additionally, this forum has lots of posts on 404 troubleshooting. Like I said, there are many different reasons for this error, and just as many fixes.