What website do you see this miniplayer on? An extension could possibly be blocking it on Edge.

Firefox has a Picture-in-Picture player but it must be opened manually by clicking the button on videos or pressing Ctrl+Shift+].

Does it still appear in Troubleshoot Mode?

I guess it could be media.hardwaremediakeys.enabled.

The dropdown video happens in troubleshooting mode too. The media.hardwaremediakeys.enabled was already set to false. Toggling it had no effect.

Is this about the mini player that some websites display if you scroll the page to keep the video in view ? If the video isn't started then you shouldn't get this mini player.

Can you post a link to a page where the media prefs aren't blocking the player ?

You can check the player in the Inspector to get its selector (#id or .classname), you would have to scroll up in the DOM to find the outer element that contains the player and add a rule to your current userContent.css to hide this player. Enclose this rule in a @-moz-document domain(){} block to apply the rule only to this domain.

The issue seems to be only with Fox website videos such as https://www.foxnews.com/video/6342556974112 and https://www.foxbusiness.com/lifestyle/video-shows-michigan-mom-punching-store-clerk-front-1-year-old-daughter-police.

media.videocontrols.picture-in-picture.enabled is set to false but picture-in-picture still pops up.

This is not about the Firefox PiP features, but is done by the website via JavaScript and CSS. There is the main .video-container DIV element and within that a DIV .sticky-wrapper that gets an extra .sticky-player once you scroll down. You can hide the .sticky-player DIV, but that leaves the .video-container DIV that has a padding-bottom property which is scrollable. I don't know hide to hide the .video-container DIV via CSS only when there is .sticky-player DIV present. This looks a bit ugly depending on the width of the window.

@-moz-document
 domain(www.foxbusiness.com),
 domain(www.foxnews.com) {
  .video-container .sticky-player {
    display:none!important;
  }
}

You can use an extension like Kill Sticky or AlwaysKillSticky to hide sticky parts of the page like miniplayers.

Though I'm not a coder and I don't know what I'm doing, I got it to work via trial and error. The first entry works so I added the other domains. Thanks for pointing me in the right direction.

@-moz-document domain(foxnews.com){

.featured.featured-video.video-ct.sticky-video .contain {
 display: none !important;
}

}

@-moz-document domain(foxbusinessnews.com){

.featured.featured-video.video-ct.sticky-video .contain {
 display: none !important;
}

}

@-moz-document domain(foxbusiness.com){

.featured.featured-video.video-ct.sticky-video .contain {
 display: none !important;
}

}