This is a feature of that website. We don't do it, so we don't have any procedure of disabling it.

Try to block it with uBlock Origin.

You can look at these prefs in about:config to see what settings work for you to block media auto-play.

• media.autoplay.default = 5 [0:allow;1:blockAudible;5:blockAll]
• media.autoplay.blocking_policy = 2
• media.autoplay.allow-extension-background-pages => false
• media.autoplay.block-event.enabled => true
• media.block-autoplay-until-in-foreground

You can open about:config via the address/location bar. You can read the warning and click "Accept the Risk and Continue".

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

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).