[saidajigumi]

That's probably because Firefox on OS X (32.0.3) doesn't yet handle H.264, which is what these GIFV files really are.

FF on Windows (32.0.2) does support H.264, and the GIFV files play as expected. IIRC, FF on Android was the first version to get H.264 support, which it's had for some time now.

EDIT: You can check codec support for your current browser+platform at YouTube's HTML5 page, below. I briefly dug around in Bugzilla for this issue, but haven't found it yet.

https://www.youtube.com/html5

[ssalenik]

You're right, the page you linked shows that my browser doesn't have support for H.264... though then I'm confused as to why the first GIFV loads for me when I open it on a separate page...