It was a YouTube update on Friday. I added support for a new layout in version 18.4.5, but the Firefox store review is so slow, and we don't have an option to speed it up.

As a workaround, before the update is available, you can use our feed by group:
https://youtu.be/r4iX53Qg2sE?si=74uy0QUoGLRmAVpB&t=232
It also has a filter by watched videos, which was not affected. Also, another browser, such as Chrome or Safari, can be a temporary solution.