If you're getting the annoying "turn your adbwocker, pwease" popup on Youtube with #Firefox, the following CSS snippet appears to hide it and not stop the video. I only tested it once, so take it with a grain of salt.

.ytd-popup-container {
        display: none !important;
}

https://cdn.masto.host/mastopt/media_attachments/files/111/172/315/472/656/211/original/8f7738d4fe7d4536.png