In earlier versions, Firefox would mark the tab that was using the "picture in picture" feature. This was VERY convenient! Imagine you had 10+ tabs open with the same domain (for example YouTube)—one of them was playing a video that you were watching via "picture in picture" while you were working on something else. In the current version of the browser, you can't quickly close the tab with the video because you first had to find it among the other tabs on the same domain. The "back to tab" option doesn't always work correctly. Not to mention, it's an unnecessary step. It was much more convenient and helpful when Firefox marked the tabs with videos playing via "picture in picture".