It was a YouTube update yesterday. I've added support for a new layout in version 18.4.5. I hope it'll be available in Firefox next week after the store review!
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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