[Silhouette]

My best guess would perhaps it's animated gifs triggering a bug in the browser for some reason?

That's plausible. Firefox has been buggy as an insect colony in recent releases, and other browsers didn't seem to have any problem with it.

In case it's useful to know: the page was also very slow to zoom at first in Firefox but then when I tried again a minute or two later it seemed to be back to normal. Again, this was a very obvious difference.