Since Firefox allows having multiple PIP-windows open, it would be nice to have an indicator-icon on the tab, in the PIP-window and taskbar.
The idea itself:
• On the left or right side of the tab, an icon that this tab is playing a video in PIP-mode.
• If multiple PIP-videos are opened, this icon changes to a number, like  for the first PIP-video,  for the 2nd video, and so on.
• Also, there should an icon be shown in the PIP-window itself, if multiple PIP-videos are open (like the number previously stated for the tabs). Maybe right next to the time if the video is paused or you mouseover the PIP-video.
• And last but not least: If there are multiple PIP-videos open, the number-icon should also be visible in the taskbar itself.
The biggest issue is to find a tab for muted videos, since you don't even have the audio-icon as wannabe-indicator. Sure, there is a back to the tab icon on the upper left window, but it would be a nice to heave feature to see, what PIP-video correspondents to what tab.
Why: Imagine having a few YouTube-tabs opened. Now try to find the correct tab without closing and reopening the PIP-window. This should help, I think.
Edit: Added some examples.