I've built a new computer, it's running Windows 8.1 and the latest version of Firefox. Not quite sure if I've always had this problem but if I'm on tumblr and there's an embedded Vine video, it doesn't play. Doesn't matter if I go directly to the same vine on the Vine website, still won't load. BUT, if I search for random vines on the Vine website, they'll play sometimes? The only time I've been able to avoid this problem is by running Safe Mode, which disables all my add-ons. But the weird thing is if I restart Firefox into "normal" mode and manually disable the add-ons, I still get the same problem. I've searched the internet, I've searched through this website. The drivers for my graphic card are the latest, everything is updated, disabled hardware acceleration just in case. I've tried installing the media feature pack but was told my system didn't meet requirements for it. I've opened up the web console and I got an error about mp4 file not loading but I get the same thing in safe mode. I can view the same vine(s) fine on my older laptop that's running Windows 7 in Firefox without going into safe mode or altering anything so I don't know anymore. It's a minor nusance and nothing groundbreaking but it's a bit of a p i t a if I have to always go into safe mode or c&p a link into Chrome just to watch a video.