Here's the bug I filed several years ago: https://bugzilla.mozilla.org/show_bug.cgi?id=1684718 - I just cc'd you

I interpreted the comments on that bug to mean that the Adafruit site has an unmuted video that happens to have a silent audio track, which isn’t strictly a muted autoplay issue. But maybe I’m wrong.

Yeah, I think there was some confusion about which video on the page was causing the issue, but the test cases I created are much simpler - just one video per page.