I’m trying to live Flash-free on my desktop. The first thing that didn’t work was Vimeo. I use Aurora, and so I set media.gstreamer.enabled to true, to turn on the gstreamer backend for the <video> tag. However, this still didn’t work. I tried installing more codec packs, but no luck. It turns out Ubuntu 13.10 comes with both gstreamer-1.0 and gstreamer-0.10, and Firefox only supports gstreamer-0.10. So I had to find the appropriate codec packs for 0.10 and install those also. Then, Vimeo worked (using H.264).
YouTube seems to work fine using WebM. :-) I do have the YouTube Flash to HTML5 addon installed so I don’t need to keep opting back in to the ‘trial’.