It looks like the implementation of HTML5 <video> in Firefox will use the native system media framework (DirectX, QuickTime, GStreamer) rather than come with any built-in codecs.
This decision has certain implications for cross-OS web video interoperability. There are very few or no formats that Windows, Mac and free Linux all can and do support.
So a Firefox codec downloader might be a good addition to core Firefox. Like the plugin finder, when it encountered media using a codec which it could not read, it would ask a codec finder service if a codec was available, but then instead of installing a browser-specific plugin it would install the codec at OS level.
So if a Windows user came across an Theora file, Firefox would (ideally) say something like “I can’t play this – get codec?”. If the user said yes, Firefox would go away, download DirectX Theora and Vorbis codecs, install them at OS level, and then start playing the video via DirectX as normal. As a side-effect, players like Windows Media Player would also gain the ability to play Theora, because it uses DirectX too.
Similarly, if a Linux user came across an MPEG4 file, a similar thing could happen (if the necessary licensing agreements were in place). There are trickier issues with binary compatibility here, but perhaps they could be overcome if the data in the codec finder service were well-maintained.
This would be a great aid to smoothing the user experience for all sorts of video on the web, and helping content authors publish without worrying too much that some people couldn’t play their format. Chris Double (the <video> guy) told me he liked the idea, but didn’t have time to code it. Do any of you? Or do you know anyone who cares about web video interoperability who does?