I don't know why this is not something you can, at the very least, configure via about:config. Sometimes I want to answer a message while I'm watching my video and ,without disabling always on top, I would need to:
Exit PnP, go to my messenger and answer, go back to video tab, either search the icon (sometimes disappears) or press the hotkey, possibly fullscreen it again and NOW I've answered my mess- wait, they replied!