Hi

This support article should help:

https://support.mozilla.org/en-US/kb/turn-picture-picture-mode

Opening a fixed mini player automatically is a feature offered by the website and is created via JavaScript if the main player is scrolled out of view. A lot of websites now have a fixed (pop-up) mini player in one the corners once you start playing a video that stays visible if you scroll the page. This website mini player shouldn't be confused with the Firefox PiP (picture-in-picture) feature (you likely see the Firefox PiP icon if you hover the mini player).

The main control switch for PiP is this pref on the about:config page. Disabling it (false) remove the PiP setting from Settings.

• media.videocontrols.picture-in-picture.enabled

The checkbox in Settings controls this pref.

• media.videocontrols.picture-in-picture.video-toggle.enabled

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Install uBlock Origin and enable all the Annoyances filters. If you still see sticky videos, you can hide them as-needed with an extension like Kill Sticky.