Idea Summary
Ideally, it should be possible to distinguish App instance visually by adding different Favicons. This option used to be there but is now deprecated with the newest APEX versions (I cannot say which version exactly).
Use Case
When working with multiple instances in parallel, it is very helpful to visually differenciate the instances in the tabs without having to click through the tabs by adding different favicons for each instance.
Preferred Solution (Optional)
Enable to define one favicon for each instance separately.