After more trials, because triggering that popup only happens occasionally, I've successfully tested the CSS to hide it and not stop the interaction with the video player.

tp-yt-iron-overlay-backdrop.opened, ytd-popup-container.ytd-app {
        display: none !important;
        z-index: -10 !important;
}

Use it with Stylus or a similar extension.

#Firefox #UserStyles #Stylus

https://cdn.masto.host/mastopt/media_attachments/files/111/175/883/865/334/516/original/9fa3bf0bdc46cc2d.mp4