Picture-in-picture window on macOS is not like any other additional window of an application

[eradicator_kai]

When PIP window is present, it cannot be switched to, or switched off, by Mission Control, or by keyboard shortcut "Move focus to next window" (which is equivalent to Windows' ALT+TAB). I, as Firefox user, want to have control over such windows via keyboard and via Mission Control. This PIP window is not even visible in Mission Control. It is a bug, I think.

[avakining]

This is intentional, I think. macOS has a way for windows to be set to stay on top. Music.app's miniplayer and TV.app's picture-in-picture also don't appear in Mission Control. This is just how PiP windows on macOS work.