This does appear to be a case of there being an actual spec issue with the image, and Firefox being more strict than Chrome. If anyone is curious I can post the exact errors (although they're rather verbose), otherwise I ran it through the AOM validator[0].
I'm finding two relevant Firefox bug about being more strict than Chrome[1][2], but neither of them seemed to cover this case specifically. I'm considering filing a bug about it, but doing so would link my bugzilla account (full name and whatnot) with my HN account.