It’s probably using WebView, or whatever it’s called where an android app brings up a browser window. If you have Firefox as your default web browser, apps will use it instead of chrome. It’s usually pretty nice, because if you have adblock in Firefox you also get adblock in the app.

It’s possible that the sign-in webpage wants to talk to the camera before returning control to the app.

If it’s trying to talk to a device over Bluetooth or USB, it’s not supported in Firefox. Mozilla refuses to implement WebUSB because they think the danger of letting people accidentally flash malware onto a physical device outweighs the benefits.

Are you able to independently confirm that the domaincheck container is listening to the right port? Eg netstat -tunlp on the host