The size of the PiP player window adjusts itself to the dimensions (height/width) of the video you play. This means that as long as the dimensions of the video stays the same then you keep your custom viewing size, but if you play a video with a different dimension then Firefox adjusts the PiP player to this video.

Thanks for your reply, however this does not appear to be the issue/problem.

As stated, this happens even when a SINGLE stream is put into PIP. The stream resolution doesn't change. The PIP window is set to a preferred size - and then, without warning, the PIP window will shrink mid-stream... it never gets bigger; and this happens during a single contiguous stream/playback.

The resultant size is approximately 320px, or there abouts, regardless of the stream, therefore it is clearly not resizing to the playback dimensions of the video as the stream is always 1080. Your explanation does not apply here, if it did, the PIP would increase to the native stream size - but it doesn't.

Even for the circumstances you describe, this should NEVER happen in the middle of stream, once it has been placed into a PIP window - otherwise it makes allowing the user to specify a size of the PIP pointless if the system is simply going to override it.

As I mentioned, there is evidently something else going wrong here as this applies to a single continuous PIP stream.