I have noticed a similar behavior on one of my machines recently. Usually it's a video (or recently played Spotify song) that gets 'stuck' there. Have you tried opening a new instance of Firefox, playing a video, then lock the computer (win + L) to see if it- at the very least- changes/updates to the newly opened video?
I have noticed a similar behavior on one of my machines recently. Usually it's a video (or recently played Spotify song) that gets 'stuck' there. Have you tried opening a new instance of Firefox, playing a video, then lock the computer (win + L) to see if it- at the very least- changes/updates to the newly opened video?