I don't know if its just me, but the GIFVs embedded on the page dont load for me, unless I click on them to open them in a page on their own, and the second one doesn't play even in this case.
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.
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...
They work for me, Firefox 32.0.3 on Arch Linux. You'll likely need some gstreamer packages installed (I think the package on Arch is gst-plugins-good for h264), same as for watching YouTube videos in HTML5 mode rather than Flash.
There's also an about:config option "media.gstreamer.enabled" which needs to be set to true, but I don't remember ever having to set this myself.
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