Hmm, today it works all of a sudden. No changes, no reboots, no new drivers, not restarted Firefox either. Just repeated what I did yesterday: open the link in a private window.

Yesterday when it didn't work I did try multiple times, and as mentioned that other site as well.

Guess Firefox on Windows has some flaky WebGPU support?

edit: must be, worked fine a couple of times, then it stopped with same error as before even after multiple attempts.