Picture-in-picture (pop-out) is a user controlled feature that launches a separate frame outside the window. More info: https://support.mozilla.org/kb/about-picture-picture-firefox
Websites are not allowed to trigger that kind of pop-out. What you're seeing is when sites monitor how far you have scrolled, and then change the position of a video player from its original location in text (static) to being fixed to the window frame (fixed) so that it can't be scrolled away any more.
Firefox doesn't have a feature to prevent sites from changing parts of their layout from position:static to position:fixed, but there might be an add-on that can do it. Some of the major ad blocking extensions also include features that target annoying page behaviors. You can search over here:
https://addons.mozilla.org/firefox/extensions/
