While Safari is supporting the <video> tag, they're doing it with patent-encumbered codecs brought to you through QuickTime. This means that Safari doesn't support the open video technology that Firefox does which today includes Theora video, Vorbis audio and the Ogg container (and will likely include Dirac and other codecs and containers in the future.)
The good news is that QuickTime is pretty extensible with plug-ins called QuickTime Components. The guys over at the Xiph Foundation (stewards of Ogg, Theora, and Vorbis) have built something appropriately named the Xiph QuickTime Components which when installed on your machine will bring Ogg, Theora, and Vorbis support to any of your QuickTime-enabled programs. This not only makes Safari compatible with Firefox's open video technology, it also brings Ogg/Theora/Vorbis encoding and decoding capabilities to iMovie, iTunes, QuickTime Player, Final Cut, and other QT-enabled programs.
I wrote a quick little howto doc over at Air Mozilla for installing the Mac decoders which will allow Air Mozilla playback in Safari. If you want to install the marginally larger full package for Mac to enable encoding (export from iMovie, Final Cut, QuickTime Pro, etc.) or you need the Windows package, you'll want to head over to the main Xiph QT Components page.
I'm also working on a short QuickTime movie/screencast that describes the simple process of adding the Xiph QT Components that can be used as an extra
src= in your
<video> element to help out your Safari users. Stay tuned for updates on that.