So it seems like it was caused by FF blocking HTML5 canvas fingerprinting. Enabling that allowed it all to load correctly. I guess I must've hit disallow without thinking when I first visited, and that response was saved even after clearing cache, so that example at least is probably my fault.

I do wonder if every user would be aware that the 'Allow site to extract canvas data?' popup can break site functionality though -- its not as though there's a warning to that effect in browser.