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),

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.