It is, but I'm using Firefox on Arch Linux and it works fine for me. H.264 has been supported in Firefox on Linux for a few versions now, although you need gstreamer installed with the relevant plugins.

Hey I'm on firefox (latest -31) on arch as well. Thing is vimeo doesn't load for me either. I do have the packages `gstreamer0.10` and `gstreamer0.10-base` installed, however in `about:plugins` in firefox, there are no plugins installed. So does this mean I have to have some such 'plugin' here for h264 to work?

Kinda. You need H.264 codec support in Gstreamer. Try installing the Gstreamer libav (an FFMPEG fork) plugin: https://www.archlinux.org/packages/?name=gst-libav

That should give your Gstreamer support for all formats libav supports.

Oh my I feel silly now, the package you linked is listed right in the optional deps for firefox! (Along with `gst-plugins-good` for h264 video too).

I remember being surprised that sites like dailymotion and vimeo still wouldn't work since I expected firefox to come with h264 support a while back (the whole cisco thing if I remember correctly). Always just attributed it to not having flash installed or whatever and shrugged my shoulders.

Anyways they work now, and at http://www.youtube.com/html5 it showed no h264 support before, but now it does. So thanks for the comment, without which I would have still been living in the dark. :)