I left mozillazine running overnight on my linux box with the new animation showing on the screen. When I came in this morning, in its place was "anim12". Reloading the page didn't help. Loading the page with Comm4.5 showed the animation was still there. scc says he's seen this on macOS as well, but didn't have to wait overnight to see it.

I'm pretty sure this is a duplicate of 6409. (see 2148 for details on black box description of 6409.) However, the description is not identical, but that could be attributed to compositor/layout changes since then (e.g. ALT text displayed instead of frozen image). I'm going to leave a note in 6409 to verify this when 6409 is verified. Pam, please feel free to resolve as Duplicate.

duplicate. -pn *** This bug has been marked as a duplicate of 6409 ***

Verifying as duplicate.